Ch. XXVII.] LAURENTIAN ROCKS. 583 



warning us of the danger of drawing any inference from mere negative 

 evidence, as to the extreme poverty of the fauna of the earlier seas. 



Recent investigations by the naturalists of the Canadian survey 

 have rendered it certain that below the level of the Potsdam sand- 

 stone there are slates and schists extending from New York to New- 

 foundland, occupied by a series of trilobitic forms similar in genera 

 though not in species to those found in the European Upper Cambrian 

 strata. 



Quebec Group. — The Dikelocephalus above mentioned is one of 

 the most striking fossils found in the limestones of Quebec, which 

 have recently attracted much attention. But there seems in these 

 limestones to be a mixed fauna, which renders it probable that the 

 Quebec group, as defined by Sir W. Logan and Mr. E. Billings, is the 

 representative of our Lower Llandeilo (Arenig) and Tremadoc groups 

 united. The characteristic graptolites lie in the upper portion, and 

 are identical with those of Skiddaw ; and the mixture of primordial 

 and Lower Silurian genera in the lower portion exactly reminds us of 

 the similar mixture in the Tremadoc slate, while, according to Mr. 

 Billings, there are many species identical with those of the calciferous 

 sand-rock, the formation which immediately overlies the Potsdam 

 sandstone and passes down into it imperceptibly. 



Huronian Series. — Next below the Upper Cambrian occur strata 

 called the Huronian by Sir "W. Logan, which are of vast thickness, 

 consisting chiefly of quartzite, with great masses of greenish chloritic 

 slate, which sometimes include pebbles of crystalline rocks derived 

 from the Laurentian formation, next to be described. Limestones 

 are rare in this series, but one band of 300 feet in thickness has been 

 traced for considerable distances to the north of Lake Huron. Beds 

 of greenstone are intercalated conformably with the quartzose and 

 argillaceous members of this series. No organic remains have yet 

 been found in any of the beds ; and whether they may be altered 

 Lower Cambrian or some still older sedimentary formation in a semi- 

 metamorphic state is uncertain. The Huronian strata are about 

 18,000 feet thick, and rest unconformably on the Laurentian, next to 

 be described. 



LAURENTIAN ROCKS. 



In the course of the geological survey carried on under the direc- 

 tion of Sir "W". E. Logan, it has been shown that, northward of the 

 river St. Lawrence, there is a vast series of crystalline rocks of gneiss, 

 mica-schist, quartzite, and limestone, more than 30,000 feet in thick- 

 ness, which have been called Laurentian, and which are already 

 known to occupy an area of about 200,000 square miles. They are 

 not only more ancient than the fossiliferous Cambrian formations 

 above described, but are older than the Huronian last mentioned, and 

 had undergone great disturbing movements before the Potsdam sand- 



