THE EARTH BEFORE THE CAMBRIAN 391 



rocks or because it has been so welded into them by heat and pressure 

 that it cannot be determined. 



Thickness. — The rocks referred to the Archaeozoic systems are 

 of great but unknown thickness. The lower limits of the system, as 

 stated, have never been observed, even where they have been cut 

 down many hundreds of feet in mountain ranges or in the great 

 " Pre-Cambrian shield " of Canada, which has apparently been re- 

 peatedly subjected to prolonged and profound erosion such perhaps 

 as few other regions of the world have experienced. 



Causes of Metamorphism and Deformation. — The cause of the 

 metamorphic character of the Archaeozoic rocks is readily understood 

 when the disturbances which have affected them are considered. 

 The Archaeozoic was a time (1) of unusual volcanic activity, as well as 

 (2) of great deformations. Moreover (3) the rock that now appears 

 at the surface was probably, for the most part, deeply buried be- 

 neath younger formations. These three factors alone would produce 

 metamorphic changes of the first order. Deformation was produced 

 in a number of ways. (1) The great masses of lavas which were in- 

 truded into the rocks caused them to fold and crumple. Moreover 

 (2) as the lava was withdrawn from below the surface and poured 

 out upon it, settling resulted which caused a further folding. These 

 elements, taken in connection with (3) the lateral pressure resulting 

 from the contraction of the interior of the earth (p. 359), must have 

 altered profoundly the original structure and composition of the rocks, 

 changing the lavas and tuffs and sedimentary rocks to schists of vari- 

 ous kinds, and the granites to gneisses and even schists. 



Conditions during the Archaeozoic Era. — A few deductions can 

 be made concerning the conditions which prevailed in this earliest 

 era. The presence of successive lava flows and of volcanic ash and 

 cinders shows that volcanoes were abundant and active, at least 

 locally. The conglomerates and shales prove that the surfaces of the 

 land were worn down by running water and that the rocks were weath- 

 ered, since the clays of which shales are formed were produced by the 

 weathering of igneous or other rocks. The presence of limestone 

 suggests the possibility that shell-bearing animals were in existence, 

 but since limestone is known to be formed by chemical precipitation 

 as well as by organic remains, the evidence is not conclusive. The 

 Grenville series of the St. Lawrence valley, estimated to be 50,000 

 feet thick, is distinctly stratified and is one of the greatest limestone 

 series in the earth's crust, a part if not all of which is believed to 



