[Ill.u.s.tration: Fig. 78.]

Where does the ma.s.s of ice increase, and where diminish?

What will be the effect of its melting?

How is it supposed that icebergs may have striated the rocky surface?

What is probably the condition of the bed of the seas over which icebergs now float?

Has the north of Europe and America been so depressed, during a period comparatively recent, as to admit of this explanation of the drift phenomena?

SECTION VI.

What is the condition of the interior of the earth with respect to heat?

How do the observations made in deep mines and wells prove this?

How far is the temperature influenced from the surface?

What is the general law of increment of temperature?

At what depth would most mineral substances be melted?

How is this conclusion confirmed?

What was probably the original state of the ma.s.s of the earth?

What other explanation may be given of this interior heat?

What is the elastic force upon which volcanic phenomena depend?

Upon what does the fluidity of lava depend?

Upon what does its porous structure, when cooled, depend?

Why are volcanoes situated near the sea?

Describe the princ.i.p.al lines of volcanic activity.

What are the forces tending to repress the elasticity of the ma.s.s below?

What will be the effect when the elastic is greater than the repressing force?

What produces the phenomena of the earthquake?

What is a volcano?

Why are volcanoes generally arranged a linear direction?

Under what circ.u.mstances will a new volcano be formed?

What instances are cited?

How is a volcanic cone formed?

Why are lateral cones produced?

How are volcanic cinders formed? Scoriae? Volcanic gla.s.s?

[Ill.u.s.tration: Fig. 79.]

Give instances of fractures as results of volcanic action. How are dikes formed?

[Ill.u.s.tration: Fig. 80.]

By what agency have the changes in the metamorphic rocks been effected?

Give the instance of metamorphic action from intrusive granite in Norway.

What instance is given as occurring in New Hampshire?

Give the experiment by which it is shown that these changes will result from a high temperature.

[Ill.u.s.tration: Fig. 81.]

What must be the condition of the lowest stratified rocks in regard to temperature?

Why is not the stratification destroyed?

What changes are produced by this high temperature?

Explain the connection of denudation and earthquake action.

What is the evidence that the surface of the earth is thrown into undulations during earthquakes?

What is the velocity of these undulations?

Give the instance which occurred in Chili.

To what parts of the earth are these undulations limited?

What condition of the surface may be regarded as resulting from this cause?

What is the cla.s.s of rocks most obviously referable to volcanic agency?

How do the trap rocks differ from ordinary lavas?

Why are they not vesicular?