Life's Dawn on Earth / Being the history of the oldest known fossil remains, and their relations to geological time and to the development of the animal kingdom

by Sir John William Dawson

In the map of the Laurentian nucleus of America ([fig. 4],) I have not inserted the Laurentian rocks believed to exist in the Rocky Mountains and other western ranges. Their distribution is at present uncertain, as well as the date of their elevation. They may indicate an old line of Laurentian fracture or wrinkling, parallel to the west coast, and defining its direction. In the map there should be a patch of Laurentian in the north of Newfoundland, and it should be wider at the west end of lake Superior.

Full details as to the Laurentian rocks of Canada and sectional lists of their beds will be found in the Reports of the Geological Survey, and Dr. Hunt has discussed very fully their chemical characters and metamorphism in his Chemical and Geological Essays. The recent reports of Hitchcock on New Hampshire, and Hayden on the Western Territories, contain some new facts of interest. The former recognises in the White Mountain region a series of gneisses and other altered rocks of Lower Laurentian age, and, resting unconformably on these, others corresponding to the Upper Laurentian; while above the latter are other pre-silurian formations corresponding to the Huronian and probably to the Montalban series of Hunt. These facts confirm Logan’s results in Canada; and Hitchcock finds many reasons to believe in the existence of life at the time of the deposition of these old rocks. Hayden’s report describes granitic and gneissose rocks, probably of Laurentian age, as appearing over great areas in Colorado, Arizona, Utah, and Nevada—showing the existence of this old metamorphic floor over vast regions of Western America.

The metamorphism of these rocks does not imply any change of their constituent elements, or interference with their bedded arrangement. It consists in the alteration of the sediments by merely molecular changes re-arranging their particles so as to render them crystalline, or by chemical reactions producing new combinations of their elements. Experiment shows that the action of heat, pressure, and waters containing alkaline carbonates and silicates, would produce such changes. The amount and character of change would depend on the composition of the sediment, the heat applied, the substances in solution in the water, and the lapse of time. (See Hunt’s Essays, p. 24.)

Plate III.

From a Photo by Weston.

Vincent Brooks, Day & Son, Lith.

WEATHERED SPECIMEN OF EOZOON CANADENSE. (ONE-HALF NATURAL SIZE.)

To face Chap. 3