SECTION III.. Of What Do Volcanic Rocks Consist?

Of what do volcanic rocks consist?

In what states are they ejected?

What are the principal varieties of lava, and how are they distinguished?

Why is the basaltic lava the last to be ejected?

How is the age of the volcanic rocks determined?

What are the three divisions of the volcanic rocks, as dependent upon age?

What is the proportion of the volcanic to other rocks?

How many active volcanoes exist?

Describe the eruptions of Kilauea.

Describe the eruption in Iceland in 1783.

What are the dimensions of Mount Ætna, and how has it been produced?

How are the tertiary lavas known to be such?

Where have they been most studied?

What is the evidence that these rocks in France are volcanic?

Have these lavas been produced within the historic period?

Were they produced at an early period in the earth's history?

Give the evidence that their activity was long-continued.

What is the form of the earlier volcanic rocks?

What circumstances distinguish the trappean from other volcanic rocks?

What are some of the prominent localities of the trappean rocks?

How do they occur in the islands west of Scotland?

How in the valley of the Connecticut river?