GRENVILLE SERIES OF EASTERN NORTH AMERICA 619 



of a study of a large area lying to the east of the Original Laurentian 

 Area and extending as far as the St. Maurice River, which flows 

 into the St. Lawrence about half-way between Quebec and Montreal. 

 Our knowledge of the Laurentian has also been very considerably ex- 

 tended by the researches of Kemp, Gushing, and others in the Adi- 

 rondack Mountains, as well as by the extended explorations of Low 

 and others in the far north and by the work of Lawson in the west. 



SIR WILLIAM Logan's work in the laurentian of Canada 



Before considering the results of these recent studies it will be of 

 interest to look very briefly at the work done by Logan in the Original 

 Laurentian Area. 



This master of stratigraphy found that the Laurentian system, 

 instead of presenting a chaotic mass of crystalline rocks from which 

 no order could be evolved, constituted as a matter of fact a great 

 series of rocks largely stratiform, if not stratified, In character, and 

 which almost everywhere exhibited a foliated structure. Among 

 these rocks he found great .bodies of hmestone, the importance and 

 significance of which he at once recognized. This hmestone was, 

 it is true, frequently very impure and always coarsely crystalline, 

 constituting in fact a true marble. Its chemical composition, however, 

 led him to conclude that it was of sedimentary origin. The lime- 

 stone occurred in the form of belts or bands, and, being easily recog- 

 nized in the field, was used by him as a basis for working out the 

 complicated stratigraphy of the district. Being soft and easily 

 disintegrated, it frequently occupies low drift-covered ground, and 

 over considerable tracts of country he was obliged to determine its 

 existence beneath the drift by driving down a sharply pointed iron 

 rod and testing the powder brought up by the rod, by means of acid. 

 He was enabled by the use of this geological cheese-taster to deter- 

 mine the existence of the limestone beneath the drift in many places 

 where its occurrence could not be otherwise ascertained. 



Associated with the limestones he found in places bands of quart- 

 zite. Bands of hornblendic rocks, now termed amphibolites, often 

 of great thickness, also formed part of the series. He ascertained, 

 however, that orthoclase gneiss was the most abundant rock in the 

 Laurentian. This, he states, varies greatly in character, being 



