48 HISTORICAL GEOLOGY 



era. In this respect, therefore, the Proterozoic was transitional 

 in character. 



"The Algonkian system as this term is used by the United 

 States Geological Survey, includes sedimentary formations and 

 their metamorphosed equivalents with associated igneous rocks 

 beneath the Cambrian and resting upon the Archean complex. 

 It includes the greater part of the sedimentary rocks of pre-Cam- 

 brian age and practically all to which present stratigraphic methods 

 have been found to apply, though it contains also sedimentary 

 rocks so deformed and metamorphosed that their stratigraphy is 

 obscure. . . . The Algonkian sediments are known to contain a 

 few fossils, representing the earliest forms of life yet found." 1 



An important feature, especially of the later Proterozoic rocks, 

 is the frequent presence of great series of non-metamorphosed 

 strata which are therefore the oldest known unaltered strata of the 

 geologic column. Such strata include all common types of sedi- 

 mentary rocks as conglomerates, sandstones, shales, and limestones. 

 Basal conglomerates, which were derived from the lands over 

 which the Proterozoic seas at various times spread or transgressed, 

 are frequently found at the bottoms of the great sedimentary series. 

 Other great series of Proterozoic rocks of undoubted sedimentary 

 origin are more or less metamorphosed to schists, quartzites, and 

 crystalline limestones. The earliest Algonkian sediments were 

 derived from exposed portions of the Archean, while later Algon- 

 kian sediments may have been derived either from exposed Ar- 

 chean or older Algonkian. That the processes of sedimentation 

 during the Proterozoic era were essentially the same as those of 

 today is clearly proved by the very character of the sediments, 

 the typical stratification to even lamination, shallow-water 

 marks, etc. 



Beside the sedimentary deposits, there is much igneous rock 

 both in the forms of intrusions into the sediments, and as extru- 

 sions or lava-flows. In the latest (Keweenawan) Algonkian series 

 of the Lake Superior district lava flows or beds predominate over 

 sediments, while in the older Algonkian series igneous rocks (either 

 intrusive or extrusive) may locally predominate. 



In addition to the frequent metamorphism, the Algonkian rocks 

 have often been subjected to great deformative movements in the 

 earth's crust so that the rocks have either been tilted or highly 

 1 Van Hise and Leith: U. S. Geol. Survey Bull. 360, 1909, p. 32. 



