632 FRANK D. ADAMS 



In. the section along the line of the Hastings road, it is estimated 

 that the "blue limestone" and the "limestone and amphibolite," 

 which represent the calcareous part of the series, contain about two- 

 thirds of their thickness of pure limestone. This would give a thick- 

 ness of 50,286 feet of pure limestone out of a total thickness of 94,406 

 feet, equal to 53 .3 per cent. This latter section, being a much longer 

 one, probably represents more nearly the average proportion of lime- 

 stone in the Grenville series as developed in this area, which in its 

 turn affords a representative area of the series as it occurs in Canada, 

 so that it may be stated that the Grenville series, as a whole, contains 

 rather more than one-half its thickness of pure limestone. The 

 thickest development of limestone in any occurrence of the Huronian 

 in America is the Randville dolomite in the Menominee Range, 

 which attains a thickness of 1,500 feet. In the Belt Formation and in 

 the series of pre-Cambrian rocks in the Lewis and Livingstone Ranges, 

 there is a thickness of limestone amounting to 4,400 and 5,400 feet 

 respectively. 



As will be seen, the thickness of limestone in the Grenville series 

 is much greater than in any of these. 



It may be safely stated that the Grenville series presents by far 

 the thickest development of pre-Cambrian limestones in North Amer- 

 ica, and that it presents at the same time one of the thickest, if not 

 the thickest, series of pre-Cambrian rocks on this continent. 



AREAL EXTENT OF THE GRENVILLE SERIES 



Not only has the Grenville series a great thickness, but it has a 

 very wide areal distribution. It is exposed along the southern border 

 of the protaxis from the Georgian Bay eastward to a point consider- 

 ably beyond the St. Maurice River, which flows into the St. Lawrence 

 at the town of Three Rivers. It extends to the northward in the 

 Laurentian Highlands at least as far as the latitude of Cobalt, although 

 not reaching so far west as this point, which lies in a Keewatin and 

 Huronian district. To the southeast it is stated by Professor H. P. 

 Gushing to be exposed at intervals over the whole Adirondack area, 

 while to the southwest deep borings under the city of Toronto have 

 brought up, from beneath the Paleozoic, cores of a white crystalline 

 limestone which evidently belongs to the same series. The area thus 



