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Updated: June 8, 2025
All forms of animal life, from the coelenterates to the mammals, follow the same path in their embryological development as far as the gastrula stage, but here their paths widely diverge, those of each subkingdom going their own separate ways. We may infer, therefore, that during the pre-Cambrian periods organic evolution followed the lines thus dimly traced.
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.
At the present time the pre-Cambrian region about Lake Superior leads the world in the production of iron ore, its output for 1903 being more than five sevenths of the entire output of the whole United States, and exceeding that of any foreign country. A theory held by many refers the ultimate source of the iron to the igneous rocks of the Archean.
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.
In the latitude of the Bow River, near the Canadian Pacific main line, there is a long narrow valley of these Cretaceous beds some sixty-five miles long, called the Cascade Trough, with of course pre-Cambrian mountains on each side.
Doubtless these remains are only the most fragmentary representation of the life of the time, for the Cambrian rocks are very old and have been widely metamorphosed. Yet the five hundred and more species already discovered embrace all the leading types of invertebrate life, and are so varied that we must believe that their lines of descent stretch far back into the pre-Cambrian past.
Hence the bottoms of these seas were sinking troughs, ever filling with waste from the adjacent land as fast as they subsided. LATE CAMBRIAN GEOGRAPHY. The formations of the Middle and Upper Cambrian are found resting unconformably on the pre-Cambrian rocks from New York westward into Minnesota and at various points in the interior, as in Missouri and in Texas.
In sketching the outlines of the continent as it was at the beginning of the Paleozoic, it must be remembered that wherever the Lower Cambrian formations now are found was certainly then sea bottom, and wherever the Lower Cambrian are wanting, and the next formations rest directly on pre-Cambrian rocks, was probably then land.
In parts, these Ordovician rocks are much folded, crushed and metamorphosed, and they are associated with schists and altered volcanic rocks which are probably pre-Cambrian. Between the eastern and central pre-Cambrian masses carboniferous rocks are found.
These marine formations of the Ordovician outcrop about the Cambrian and pre-Cambrian areas, and, as borings show, extend far and wide over the interior of the continent beneath more recent strata. The Ordovician sea stretched from Appalachia across the Mississippi valley. It seems to have extended to California, although broken probably by several mountainous islands in the west.
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