Pre-Cambrian fossils.  Very rarely has any clear trace of an organism been found in the most ancient chapters of the geological record, so many of their leaves have been destroyed and so far have their pages been defaced.  Omitting structures whose organic nature has been questioned, there are left to mention a tiny seashell of one of the most lowly types,—­a DISCINA from the pre-Cambrian rocks of the Colorado Canyon,—­and from the pre-Cambrian rocks of Montana trails of annelid worms and casts of their burrows in ancient beaches, and fragments of the tests of crustaceans.  These diverse forms indicate that before the Algonkian had closed, life was abundant and had widely differentiated.  We may expect that other forms will be discovered as the rocks are closely searched.

Pre-Cambrian geography.  Our knowledge is far too meager to warrant an attempt to draw the varying outlines of sea and land during the Archean and Algonkian eras.  Pre-Cambrian time probably was longer than all later geological time down to the present, as we may infer from the vast thicknesses of its rocks and the unconformities which part them.  We know that during its long periods land masses again and again rose from the sea, were worn low, and were submerged and covered with the waste of other lands.  But the formations of separated regions cannot be correlated because of the absence of fossils, and nothing more can be made out than the detached chapters of local histories, such as the outline given of the district about Lake Superior.

The pre-Cambrian rocks show no evidence of any forces then at work upon the earth except the forces which are at work upon it now.  The most ancient sediments known are so like the sediments now being laid that we may infer that they were formed under conditions essentially similar to those of the present time.  There is no proof that the sands of the pre-Cambrian sandstones were swept by any more powerful waves and currents than are offshore sands to-day, or that the muds of the pre-Cambrian shales settled to the sea floor in less quiet water than such muds settle in at present.  The pre-Cambrian lands were, no doubt, worn by wind and weather, beaten by rain, and furrowed by streams as now, and, as now, they fronted the ocean with beaches on which waves dashed and along which tidal currents ran.

Perhaps the chief difference between the pre-Cambrian and the present was the absence of life upon the land.  So far as we have any knowledge, no forests covered the mountain sides, no verdure carpeted the plains, and no animals lived on the ground or in the air.  It is permitted to think of the most ancient lands as deserts of barren rock and rock waste swept by rains and trenched by powerful streams.  We may therefore suppose that the processes of their destruction went on more rapidly than at present.

CHAPTER XVI