CHAPTER XII.. Trap-Pebbles of the Boulder--Thickness of the Earth's Crust

Trap-pebbles of the boulder--Thickness of the earth's crust unknown--Not of much consequence to the practical geologist--Interior of the earth in a highly heated condition--Proofs of this--Granite and hypogene rocks--Trap-rocks: their identity with lavas and ashes--Scenery of a trappean country--Subdivisions of the trap-rocks--Intrusive traps--Trap-dykes--Intrusive sheets--Salisbury Crags--Traps of the neighbourhood of Edinburgh--Amorphous masses--Contemporaneous trap-rocks of two kinds--Contemporaneous melted rocks--Tests for their age and origin--Examples from neighbourhood of Edinburgh--Tufas or volcanic ashes--Their structure and origin--Example of contemporaneous trap-rocks--Mode of interpreting them--Volcanoes of Carboniferous times--Conclusion,

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THE STORY OF A BOULDER.