2 C. E. VAN HISE THE PROBLEM OF THE PRE-CAMBRIAN 



Cambriaii there is an unconforinit}^ This unconformity for a large part 

 of the United States is profound, being the break at the base of the great 

 Cambrian transgTession. 



In the early days of geology in America the name "primary" or "prim- 

 itive" was given to the group of rocks under consideration. By the men 

 who used these terms there was scarcely any attempt to apply strati- 

 graphic methods to them. They were content with general lithological 

 descriptions of the rocks observed. 



The original Laurentian District 



The working out of the stratigraphy of the pre-Cambrian of America 

 began when Sir William Logan came to this country, in 1842. Before 

 he took charge of the Canadian Geological Survey, Logan had had long 

 practice in working out the difficult stratigraphy of the coal fields of 

 Wales. He was above all else a stratigraphical geologist. 



Logan had not been long in Canada before he saw that antedating the 

 fossiliferous rocks are other sedimentary rocks, to which he applied the 

 methods he had so successfully used in Wales. The first region in which 

 this attempt was made was the Laurentian mountains of Canada. As a 

 result of Logan's studies, extending through a number of years, he 

 finally proposed in 1854 to designate the rocks of these mountains by the 

 name "Laurentian." Later Logan appreciated that this Laurentian con- 

 sisted of two parts — an upper, subordinate portion, containing limestone 

 and gneissic rocks, presumably of sedimentary origin, and a lower, domi- 

 nant mass, consisting mainly of various gneisses and granites. Still 

 later an anorthosite formation was recognized which was regarded as 

 newer than the formations mentioned, so it was called LTpper Laurentian. 

 The term Grenville was introduced by Logan in 1863 to designate the 

 upper sedimentary limestone series, although he still used the term Lau- 

 rentian system to cover all of the formations of the district. 



Later work by Adams has shown the anorthosite series to be intrusive 

 in the older rocks. He has also shown that some of the gneisses asso- 

 ciated with the limestone of the Grenville series have the composition of 

 sediments, and that some of the Lower Laurentian gneisses and granites 

 have the composition of igneous rocks; and, further, that many of the 

 Lower Laurentian gneisses and granites intrude the Grenville sediments. 



The Hastings District 



Southwest of the original Laurentian area and north of the east end of 

 lake Ontario is the so-called Hastings series, in which work was first done 



