Eclectic Physical Geography (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1888 edition. Excerpt: ... many thousands or even millions of years have been required for these rocks to accumulate. There is no way to determine the exact length of geological time. Some circumstances indicate that at least 3,000,000 years must have elapsed since the oldest known sedimentary rocks were deposited; other circumstances indicate that it could not have been more than 100,000,000 years, but neither of these estimates is accurate--the time may be greater, or it may be less. All that can safely be affirmed is that the fragmentary record of the earth's history which the sedimentary rocks afford, covers a very long period of time. It is perhaps impossible for the human intellect to grasp the lapse of time comprehended in the expression "one million years." By a great effort of memory, an old man may appreciate the length of not much more than half a century; and yet if half a century be represented by a distance of three inches, a million years would be represented by one mile. CHAPTER XIV. THE WATER OF THE LAND SPRINGS. He sendeth the springs into the valleys, which run among the hills. They give drink to every beast of the field.--Psalm Civ: io, n. The vapor of the atmosphere, through its condensation into rain, snow, dew, etc., supplies all the water encountered on the surface of the land. This may be classified according to its manner of occurrence, as springs, streams, glaciers, and lakes. Permeability of Rocks.--All rocks can absorb more or less water. Clay and fine grained, compact rocks, though they may contain water, do not allow it to escape readily, and are therefore said to be impermeable. A layer of soil, sand, or coarse grained, loosely cohering, or much fissured rock, on the contrary, allows water to pass through it copiously, or...