CHAPTER IV.

What is the object of the preceding chapters?

How can we arrive at a knowledge of the causes which have produced geological phenomena?

Have geological causes always operated with the same intensity?

How are the means of forming correct geological theories increasing?

SECTION I.

How does oxygen become an agent in the disintegration of rocks?

How does carbonic acid operate in the disintegration of rocks?

What is the effect of moisture and rain?

What is the effect of variations of temperature?

What other atmospheric causes are mentioned?

How do these causes become important?

What are some of their effects?

SECTION II.

What are the changes which are to be referred to chemical agency?

Mention some of the disturbances which give rise to chemical changes.

What are the principal effects of chemical action?

How is the cleavage structure accounted for?

Mention instances which show that a cleavage may be established in a body in a solid state.

In it a crystalline arrangement of the particles of the mass?

What other divisional planes exist in rocks?

Mention instances of concretionary formations.

Why may not these concretions have been deposited as nodules?

How have these concretions been formed?

Mention instances of segregation without the concretionary structure.

How was the segregation in these instances effected?

How is the columnar structure produced?

What is the origin of the mineral veins which are first mentioned?

How is it shown that other veins are not injected?

How were these veins formed?

What is the force by which these molecular changes have been effected?

SECTION III.

In what ways are geological changes produced by human agency?

Of what are the organic remains, in rocks, the record?

What rocks contain organic materials in large quantity?

What is the most abundant organic product?

Explain the mode of growth of corals.

Give instances of extensive coral reefs.

What is the total amount of surface covered by the coral reefs?

SECTION IV.

What degree of importance is attached to water as a geological agent?

What are the sources of the sediment which water deposits?

Why is not the formation of the sedimentary rocks capable of being observed?

What is the first mode in which solid matter is taken up by water?

Why are the waters of the ocean saline?

What effect has the temperature of water in the solution of silex?

What effect has an alkaline condition of water?

What rock is soluble in water charged with carbonic acid?

Give an instance of limestone formation from such solutions.

How do rivers furnish sediment for the stratified rocks?

What determines the position of rapids in rivers?

What is the effect of waterfalls on the abrading action of rivers?

What is the peculiarity of rock at Niagara which has prevented the fall from becoming a succession of rapids?

What other circumstance increases the abrading action of rivers?

What is the principal source of the sediment which is transported by rivers?

What is the annual amount of sediment furnished by the Kennebec? The Merrimac? The Mississippi? The Ganges?

What is the general tendency of these abrading forces?

What is the effect of waves upon the coast, when it consists of unsolidified materials?

Describe their effect upon rocky coasts.

How is the encroachment upon such coasts shown?

What is the effect of waves of less power?

How are marine currents produced?

How are they increased by the evaporation of the torrid zone?

What are the most important marine currents?

Which class of currents have the greater depth?

Upon what does the power of deep currents depend?

How would the effect of these currents be increased by earthquakes?

Where will the effects of these currents be greatest?

Mention instances of these effects.

What must be the effect of such currents as the Gulf-stream and Mozambique channel?

Mention, generally, the effects of these currents.

Why does detrital matter remain suspended in the water of rivers?

How is the coarse and fine sediment separated?

Why do river currents extend some distance into the sea?

What effect does this have in distributing the sediment which the rivers furnish?

Upon what does the transporting power of marine currents depend?

When a river enters a lake, why is its sediment deposited?

Describe the effect.

When is sediment deposited in the beds of rivers?

Describe the effects of this deposition.

Where is most of the sediment deposited?

Give the area of some delta deposits.

How do the deep-sea deposits now forming compare in extent with the earlier formations?

State the several circumstances by which a succession of deposits would be arranged in strata.

How are those differences produced upon which the separation into independent formations depends?

Why are marine deposits nearly horizontal?

How are the irregular stratifications produced?

What peculiarity in the fossils will distinguish the lacustrine and marine deposits?

What peculiarity in reference to fossils will characterize the deep-sea deposits?

How is coal shown to be of vegetable origin?

Why will the drift wood of the sea accumulate in particular localities, and why will it sink?

Why will it become buried beneath earthy matter?

How is it known that wood thus buried will, at length, become lignite?

How is lignite converted into mineral coal?

What is the proof of it?

Have beds of coal been formed at other periods, besides the carboniferous?

Is it probable that coal-beds are now forming?

How did the flora of the carboniferous period differ from the existing flora?

Are the alternations of the earthy and coal strata satisfactorily explained?

In what portions of the geological series are the deposits of salt found?

Where is saline matter principally stored?

Explain the conjectural formation of salt in the Mediterranean Sea.

What form do rocks take when deposited from a chemical solution?

How is sand or gravel solidified by the infiltration of mineral waters?

What is the effect of drying upon the solidification of rocks?

What is the effect of pressure?

What of heat?

SECTION V.

What is a glacier?

What is the extent of the glaciers of the Alps?

What change does the mass of snow in the higher valleys of the glacier mountains undergo?

What is the source of supply to the glacier?

What is the cause of the motion of the glacier?

What is the usual annual motion?

Why will the glacier melt but little at its under side?

Where will the waste at the surface just equal the addition?

What circumstances vary the position of the terminus of the glacier?

What, besides snow and ice, enters into the composition of a glacier?

How are these materials supplied?

How is a lateral moraine formed?

What effect has the motion of the glacier on the rocky surface over which it passes?

What is the material by which this effect is produced?

How is the terminal moraine produced?

How may the moraines on the Jura Mountains be explained?

How has it been proposed to explain the striated surfaces of rocks found in the north of Europe and America?

What is the objection to this extension of the glacier theory?

How does the ice accumulate along the coast in high latitudes to form icebergs?

Why does it ultimately separate from the shore?

How does it become freighted with earthy matter?

In what direction do the icebergs float, and why?

What are the dimensions of an iceberg, estimated from the part that is visible?

Where does the mass 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 mass 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 principal lines of volcanic activity.

What are the forces tending to repress the elasticity of the mass 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 circumstances 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? Scoriæ? Volcanic glass?

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

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.

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 class of rocks most obviously referable to volcanic agency?

How do the trap rocks differ from ordinary lavas?

Why are they not vesicular?

Why more crystalline?

Why were cones never formed?

What is the proof that the granitic rocks have once been in a melted state?

Why does not the mass of melted rock below the surface retain permanently its liquid form?

Why does it, on cooling, become more crystalline than lava?

State the process by which mountains are formed.

By what law does the elevating force accumulate?

Why, then, is the process of elevation spasmodic, and not constant?

How is the inclined position of strata produced?

How are strata brought into a vertical position over large areas?

Why do subsidences occasionally follow these movements of elevation? Mention instances.

What explanation is suggested of deep and extensive chasms?

What conditions must exist together, in the force by which continents are produced?

What cause fulfils these conditions?

What is the proof that the temperature under given localities is variable?

What will be the result of these variations?

What is the law of expansion of rocks, as obtained by experiment?

What amount of change of level may be thus accounted for?

What circumstances would probably increase this amount?

What amount of vertical movement must be accounted for?

Why must these changes of level be very slow?

Under what conditions would there be no change of level?

Is it probable that these conditions exist to any great extent?

Why, then, are not the changes of level observed?

Why is the bed of the sea most likely to experience the change of elevation?

Why are the continents most favorably situated to undergo depression?

What are the sources of heat upon which climate depends?

Does the interior temperature sensibly affect the present climates?

What cause may be assigned for the changes of climate which are known to have taken place?

What are the relations of land by which the highest temperature would be produced?

How would this distribution of land affect the temperature of the waters of the ocean?

What would result if the opposite relations of land and water existed?

What confirmation of these conclusions is drawn from the existing climates of different parts of the earth?

Is there any reason to suppose that the relations of land and water which would have produced a warmer climate in former times did not exist?