6 C. E. VAX HISE THE PR0BLE:M OF THE PEE-CAMBRIAN 



the Huronian to be iui,conformably below the Huronian, and to be in- 

 truded by the granite, and therefore in age more nearly related to the 

 Lanrentian than to the Huronian. 



Finalh^, north of Sudbury, some distance north of lake Huron, is an- 

 other series, which Bell placed as Cambrian on the Canadian maps, but 

 which is now agreed to be pre-Cambrian and which our studies show 

 probably to be imconformably above the Huronian immediately north of 

 lake Huron, mapped in detail by Logan, but in Logan's general map in- 

 cluded in the Huronian. 



Thus, in the region north of lake Huron are two groups of rocks sepa- 

 rated by imconformities. The lower consists subordinately of basic vol- 

 canic and dominantly of granitic and granitoid gneisses, the acid rocks 

 being intrusive in the basic. According to the usage advocated by the 

 International Committee, on the Lake Superior region below mentioned 

 the volcanics are Keewatin and the granites and gneisses Lanrentian. 

 The upper group comprises three tinconformably sedimentary series, 

 which ma}" be called Lower, Middle, and Upper Huronian. 



Later TToek of Logax axd Others 



The work of Logan in the Lanrentian mountains and of Logan and 

 jMurray on the north shore of lake Huron occupied many years, yet the 

 areas of the districts mapped were almost infinitesimal as compared with 

 the vast expanses of pre-Cambrian rocks of Canada. In this early care- 

 ful detailed work the rocks which had been called Lanrentian were mainly 

 light colored, pink or gray with intermediate shades, of the granitic 

 order, or, as we would perhaps say at present, gneissoid granites and 

 granitic gneisses. The Huronian series of rocks were largely composed 

 of green and dark gray sediments with intermediate shades and subordi- 

 nate masses of light colored quartzites and limestones, but gray and green 

 were the prevailing tones: also it is to be remembered that Logan placed 

 with these series green chloritic volcanic rocks, which later work has 

 shown to he unconformably below them. 



A period of rapid reconnaissance mapping in Canada now followed the 

 early epoch of close systematic work upon confined areas. The colors 

 gave the clew to the geologist. Here was an easier method tlian digging 

 out the stratigraphy from the forests, which method required years of 

 laborious work for small areas. The pink and gray rocks of granitic 

 t}'pes were Lanrentian, and the dark gray and green rocks, whether sedi- 

 mentary or igneous, were Huronian. Thus was inaugurated the chro- 

 matic epoch of mapping the jJre-Cambrian formations, which is at least 

 a score times as rapid and easy as the early methods of Logan and Murray 



