I now bring our study of the aqueous or superficial agencies to a conclusion by noting once more that the great geological results accomplished by _air_, _water_, and _organic matter_ or _life_ are: (1) _Erosion_, or the wearing away of the surface of the land; and (2) _Deposition_, or the formation from the débris of the eroded land of two great classes of stratified rocks,—the mechanically formed or fragmental rocks, and the chemically and organically formed rocks. II. IGNEOUS AGENCIES. We pass next to a very brief consideration of operations that originate below the earth’s surface. The records of deep mines and artesian wells show that the temperature of the ground always increases downwards from the surface; and the much higher temperatures of hot springs and volcanoes show that the heat continues to increase to a great depth, and is not a merely superficial phenomenon. The observed rate of increase is not uniform, but it seldom varies far from the average, which is about 1° Fahr. per 53 feet of vertical descent, or, in round numbers, 100° per mile. This rate, if continued, would give a very high temperature at points only a few miles below the surface; and until within a few years the idea was generally accepted by geologists that the increase of temperature is sensibly uniform for an indefinite distance downward; that in the central regions of the earth the temperature is far higher than anything we can conceive, and that everywhere below a depth of 20 to 40 miles the temperature is above the fusing-point of all rocks; and hence that the earth is an incandescent liquid globe covered by a thin shell or crust of cold, solid rock. Our limited space will not permit us to enter into a discussion of the condition of the earth’s interior, and I will merely point out in a few sentences the position occupied by geologists at the present time. The reasoning of Thompson has shown that the temperature cannot increase downward at a uniform rate, but at a constantly and rapidly diminishing rate; and that everywhere below a depth of 300 miles the temperature is probably sensibly the same, and nowhere, probably, above 8000° to 10,000° Fahr.