Part 31

Sabine, Sir Edward, Address of the president of the Royal Society, this Journal, =37=, 108, 1864.

IV A CENTURY OF GEOLOGY.—THE GROWTH OF KNOWLEDGE OF EARTH STRUCTURE

By JOSEPH BARRELL

Introduction The Intellectual Viewpoint in 1818.

In 1818, the year of the founding of the Journal, the natural sciences were still in their infancy in Europe. Geology was still subordinate to mineralogy, was hardly recognized as a distinct science, and consisted in little more than a description of the character and distribution of minerals and rocks. America was remote from the Old World centers of learning. The energy of the young nation was absorbed in its own expansion, and but a few of those who by aptitude were fitted to increase scientific knowledge were even conscious of the existence of such a field of endeavor. Under these circumstances the educative field open to a journal of science in the United States was an almost virgin soil. Original contributions could most readily be based upon the natural history of the New World, and the founder of the Journal showed insight appreciative of the situation in stating in the “Plan of the Work” in the introduction to the first volume that “It will be a leading object to illustrate AMERICAN NATURAL HISTORY, and especially our MINERALOGY and GEOLOGY.”

At this time educated people were still satisfied that the whole knowledge of the origin and development of the earth so far as man could or should know it was embraced in the Book of Genesis. They were inclined to look with misgiving at attempts to directly interrogate the earth as to its history. Philosophers such as Descartes and Liebnitz, the cosmogonists de Maillet and Buffon had been less instrumental in developing science than in fitting a few facts and many speculations to their systems of philosophy. By the opening of the nineteenth century, however, men of learning were coming to appreciate that the way to advance science was to experiment and observe, to collect facts and discourage unfounded speculation. Silliman’s insight into the needs of geologic science is shown in the following quotation (=1=, pp. 6, 7, 1818):

“Our geology, also, presents a most interesting field of inquiry. A grand outline has recently been drawn by Mr. Maclure, with a masterly hand, and with a vast extent of personal observation and labour: but to fill up the detail, both observation and labour still more extensive are demanded; nor can the object be effected, till more good geologists are formed, and distributed over our extensive territory.

To account for the formation and changes of our globe, by excursions of the imagination, often splendid and imposing, but usually visionary, and almost always baseless, was, till within half a century, the business of geological speculations; but this research has now assumed a more sober character; the science of geology has been reared upon numerous and accurate observations of facts; and standing thus upon the basis of induction, it is entitled to a rank among those sciences which Lord Bacon’s Philosophy has contributed to create. Geological researches are now prosecuted by actually exploring the structure and arrangement of districts, countries, and continents. The obliquity of the strata of most rocks, causing their edges to project in many places above the surface; their exposure, in other instances on the sides or tops of hills and mountains; or, in consequence of the intersection of their strata, by roads, canals, and river-courses, or by the wearing of the ocean; or their direct perforation, by the shafts of mines; all these causes, and others, afford extensive means of reading the interior structure of the globe.

The outlines of American geology appear to be particularly grand, simple, and instructive; and a knowledge of the important facts, and general principles of this science, is of vast practical use, as regards the interests of agriculture, and the research for useful minerals. Geological and mineralogical descriptions, and maps of particular states and districts, are very much needed in the United States; and to excite a spirit to furnish them will form one leading object of this Journal.”

The Prolonged Influence of Outgrown Ideas.

Those interested in any branch of science should, as a matter of education, read the history of that special subject. A knowledge of the stages by which the present development has been attained is essential to give a proper perspective to the literature of each period. Much of the existing terminology is an inheritance from the first attempts at nomenclature, or may rest upon theories long discarded. Popular notions at variance with advanced teaching are often the forgotten inheritance of a past generation.

Gneiss, trap, and Old Red Sandstone are names which we owe to Werner. The “Tertiary period” and “drift” are relics of an early terminology. The geology of tourist circulars still speaks of canyons as made by “convulsions of nature.” Popular writers still attribute to geologists a belief in a molten earth covered by a thin crust. Within the present century the eighteenth century speculations of Werner and his predecessors, postulating a supposed capacity of water to seep through the crust into the interior of the earth, resulting in a hypothetical progressive desiccation of the surface, views long abandoned by most modern geologists, have been revived by an astronomer into a theory of “planetology.”

A review of the literature of a century brings to light certain tendencies in the growth of science. Each decade has witnessed a larger accumulation of observed facts and a fuller classification of these fundamental data, but the pendulum of interpretative theory swings away from the path of progress, now to one side, now to the other, testing out the proper direction. For decades the understanding of certain classes of facts may be actually retrogressive. A retrospect shows that certain minds, keen and unfettered by a prevailing theory, have in some directions been in advance of their generation. But the judgment of the times had not sufficient basis in knowledge for the separation and acceptance of their truer views from the contemporaneous tangle of false interpretations.

An interesting illustration of these statements regarding the slow settling of opinion may be cited in regard to the significance of the dip of the Triassic formations of the eastern United States. The strata of the Massachusetts-Connecticut basin possess a monoclinal easterly dip which averages about 20 degrees to the east. Those of the New Jersey-Pennsylvania-Virginia basin possess a similar dip to the northwest. Both basins are cut by great faults and the dip is now accepted by practically all geologists as due to rotation of the crust blocks away from a geanticlinal axis between the two basins. Edward Hitchcock, whose work from the first shows an interpretative quality in advance of his time, states in 1823 (=6=,74) regarding the dip of the Connecticut valley rocks:

“There is reason to believe that Mount Toby, the strata of which are almost horizontal, exhibits the original dip of these rocks, and that those cases in which they are more highly inclined are the result of some Plutonian convulsion. Such irregularity in the dip of coal fields is no uncommon occurrence.”

In Hitchcock’s Geology of Massachusetts, published in 1833, ten years later, geological structure sections of the Connecticut Valley rocks are given, the facts are discussed in detail and the dip ascribed to the elevatory forces. He says (l. c., pp. 213, 223):

“If it were possible to doubt that the new red sandstone formation was deposited from water, the surface of some of the layers of this shale would settle the question demonstrably. For it exhibits precisely those gentle undulations, which the loamy bottom of every river with a moderate current, presents. (No. 198.) But such a surface could never have been formed while the layers had that high inclination to the horizon, which many of them now present: so that we have here, also, decisive evidence that they have been elevated subsequently to their deposition....

The objection of a writer in the American Journal of Science, that such a height of waters as would deposit Mount Toby, must have produced a lake nearly to the upper part of New Hampshire, in the Connecticut Valley, and thus have caused the same sandstone to be produced higher up that valley than Northfield, loses its force, when it is recollected that this formation was deposited before its strata were elevated. For the elevating force undoubtedly changed the relative level of different parts of the country. In this case, the disturbing force must have acted beneath the primary rocks. And besides, we have good evidence which will be shown by and by, that our new red sandstone was formed beneath the ocean. We cannot then reason on this subject from present levels.”

In 1840, H. D. Rogers, a geologist who has acquired a more widely known name than Hitchcock, but who in reality showed an inferior ability in interpretation, made the following statements in explanation of the regional monoclinal dip of the New Jersey Triassic rocks averaging 15 to 20 degrees to the northwest:

“Their materials give evidence of having been swept into this estuary, or great ancient river, from the south and southeast, by a current producing an almost universal dip of the beds towards the northwest, a feature clearly not caused by any uplifting agency, but assumed originally at the time of their deposition, in consequence of the setting of the current from the opposite or southeastern shore.”

In 1842, at the third annual meeting of the Association of American Geologists both H. D. and W. B. Rogers argued (=43=, 170, 1842) against Sir Charles Lyell and E. Hitchcock that the present dip of the Triassic was the original slope of deposition, stating among other reasons that the footprints impressed upon the sediments often showed a slipping and a pushing of the soft clay in the direction of the downhill slope. In 1858 H. D. Rogers still held to the same views of original dip, notwithstanding that a moderate amount of observation on the mud-cracked and rain-pitted layers would have supplied the proof that such must have dried as horizontal surfaces. The idea of inclined deposition is not yet wholly dead as it has been suggested more than once within the present generation as a means of escaping from the necessity of accepting the very great thicknesses of this and similar formations. Thus, as Brögger has remarked in another connection,—the ghosts of the old time stand ever ready to reappear.

In the present essay on the rise of structural geology as reflected through a century of publication in the Journal, attention will be given especially to two fields, that of structures connected with igneous rocks and that of structures connected with mountain making, and emphasis will be placed upon the growth of understanding rather than upon the accumulating knowledge of details. The growth in both of these divisions of structural geology is well illustrated in the volumes of the Journal.

Structures and Relationships of Igneous Rocks.

Opposed Interpretations of Plutonists and Neptunists.

During the first quarter of the nineteenth century the geologic controversy between the Plutonists and Neptunists was at its height; the Plutonists, following the Scotchman, Hutton, holding to the igneous origin of basalt and granite, the Neptunists, after their German master, Werner of Freiberg, maintaining that these rocks had been precipitated from a primitive universal ocean. The Plutonists, although time has shown them to have been correct in all essential particulars, were for a generation submerged under the propaganda carried forward by the disciples of Werner. The “Illustrations of the Huttonian Theory of the Earth,” a remarkable classic, worthy of being studied to-day as well as a century ago, was published in 1802 by John Playfair, professor of mathematics in the University of Edinburgh and a friend of Hutton, who had died five years previously. This volume was opposed by Robert Jameson, professor of natural philosophy in the same university, who had absorbed the ideas of the German school while at Freiberg and published in 1808 a volume on the “Elements of Geognosy,” in which the philosophy of Werner is followed throughout and even obsidian and pumice are argued to be aqueous precipitates. The authority of the Wernerian autocracy caused its nomenclature to be adopted in the new world, but strong evidence against its interpretations was to be found in the actual structural relations displayed by the igneous rocks.

Contributions on Volcanic and Intrusive Rocks.

The accumulation and study of facts constituted the best cure for an erroneous theory. The publications of the Journal contributed toward this end by articles along several lines. The most original contributions were those which dealt with the areal and structural geology of eastern North America, but equally valuable at that time for the broadening of scientific interest were the studies on the volcanic activities of the Hawaiian Islands, published through many years. Perhaps most valuable from the educative standpoint were the extensive republications in the Journal of the more important European researches, making them accessible to American readers. In volume =13= (1828), for example, a digest of Scrope’s work on volcanoes is given, covering forty pages; and of Daubeny on active and extinct volcanoes, running over seventy-five pages and extending into vol. =14=. Through these comprehensive studies the nature of volcanic action became generally understood during the first half of the nineteenth century and the original publications in the Journal were valuable in giving a knowledge of the activities of the Hawaiian volcanoes.

Early in the nineteenth century the whole of America still remained to be explored by the geologist. The regions adjacent to the centers of learning were among the first to receive attention and the Triassic basin of Connecticut and Massachusetts yielded information in regard to the nature of igneous intrusion. This basin, of unmetamorphic shales and sandstones, is occupied by the Connecticut River except at its southern end. The Formation contains within it sills, dikes, and outflows of basaltic rocks which because of their superior resistance to erosion constitute prominent hills, in places bounded by cliffs.