Common Minerals and Rocks

by William O. Crosby

Fig. 8.—Ripple-marks in sandstone.

Rill-marks, Rain-prints, and Sun-cracks.—“One of the most fascinating parts of the work of a field-geologist consists in tracing the shores of former seas and lakes, and thus reconstructing the geography of successive geological periods.” His conclusions, as we have already seen, are based largely upon the nature of the sediments; but still more convincing is the evidence afforded by those superficial features of the strata, which, like ripple-marks, seem, by themselves, quite insignificant. And among these he lays special emphasis upon those which show that during their deposition strata have at intervals been laid bare to sun and air.

During ebb tide water which has been left at the upper edge of the beach runs down across the beach in small rills, which excavate miniature channels; and when these are preserved in the hard rocks, they prove that the latter are beach deposits, and, like the ripple-marks, show the direction of the old shore.

If a heavy shower of rain falls on a muddy beach or flat, the sediment deposited by the returning tide may cover, without obliterating, the small but characteristic impressions of the individual drops; and these markings are frequently found well preserved in the hardest slates and sandstones, testifying unequivocally to the conditions under which the rocks were formed. In some cases the rain-prints are found to be ridged up on one side only, in such a manner as to indicate that the drops as they fell were driven aslant by the wind. The prominent side of the marking, therefore, indicates the side towards which the wind blew.

Muddy sediments, especially in lakes and rivers, are often exposed to the air and sun during periods of drouth, and as they gradually dry up, polygonal cracks are formed. The sediment of the next layer will fill these sun-cracks; and when, as often happens, it is slightly different from the dessicated layer, they may still be traced. Sun-cracks preserved in this way are very characteristic of argillaceous rocks, and, of course, prove that in early times, as at the present day, sediments of this class were exposed by the temporary retreat of the water. The foot-prints or trails of land-animals are often, as in the sandstones and shales of the Connecticut Valley, associated with, and of course strongly corroborate, all these other evidences of shore deposits. From the foot-prints preserved in the rocks we pass naturally to the consideration of the fossil remains of plants and animals found entombed in the strata.

Fossils.—Although fossils find their highest interest in the light which they throw upon the succession of life on the globe, they may also be properly regarded as structural features of stratified rocks; and any one who has seen the dead shells, crabs, fishes, etc., on the beach will readily understand how fossils get into the rocks. It is not our province here to study the structure of the fossils themselves, for that would involve us in a course in paleontology, a task belonging to the biologist rather than the geologist; but we will merely observe the three principal degrees in the preservation of fossils:—

1. Original composition not completely changed.—Extinct elephants have been found frozen in the river-bluffs of Siberia so perfectly preserved that dogs and wolves ate their flesh. The bodies of animals are also found well preserved in peat-bogs. All coal is simply fossil vegetation retaining in a large degree the original composition; and the same is true of ferns, etc., preserved as black impressions in the rocks. All bones and shells consist of mineral matter which makes them hard, and animal matter which makes them tough and strong. In very many cases, especially in the newer formations, the animal matter is still partially, and the mineral matter almost wholly, intact.

2. Original composition completely changed, but form and structure preserved.—All kinds of fossils are commonly called petrifactions, but only those preserved in this second way are truly petrified, i.e., turned to stone. “Petrified wood is the best illustration, and in a good specimen not only the external form of the wood, not only its general structure—bark, wood, radiating silver-grain, and concentric rings of growth—are discernible, but even the microscopic cellular structure of the wood, and the exquisite sculpturing of the cell-walls, are perfectly preserved, so that the kind of wood may often be determined by the microscope with the utmost certainty. Yet not one particle of the organic matter of the wood remains. It has been entirely replaced by mineral matter; usually by some form of silica. The same is true of the shells and bones of animals.”—Le Conte.

3. Original composition and structure both obliterated, and form alone preserved.—This occurs most commonly with shells, although fossil trees are also often good illustrations. The general result is accomplished in several ways: (a) The shell after being buried in the sediment may be removed by solution, leaving a mould of its external form, (b) This mould may subsequently be filled by the infiltration of finer sediment, forming a cast of the exterior of the shell. (c) The shell, before its solution, may have been filled with mud; and if the shell itself is then dissolved, we have a cast of its interior in a mould of its exterior.

Time required for the Formation of Stratified Rocks.—Many attempts have been made to determine the time required for the deposition of any given thickness of stratified rocks. Of course, only roughly approximate results can be hoped for in most cases; but these are at least sufficient to make it certain that geological time is very long. The average relative rate of growth of different kinds of sediment is, however, less open to doubt, for we have already seen that coarse sediments like gravel and sand accumulate much more rapidly than finer sediments like clay and limestone; and we are sometimes able to compare these two classes of rocks on a very large scale.