A SKETCH OF GEOLOGY. 17 



All the different metamorphic rocks constitute together the Archaean forma- 

 tion, which is subdivided into the Laurentian and Huronian groups. The 

 Laurentian group forms the base of the whole geological column of sediment- 

 ary rocks. It derives its name either from the St. Lawrence river or from the 

 Laurentian mountains in Canada, where it is exposed over an extensive area. 

 In the United States we find it cropping out in New York, where it forms the 

 Adirondack mountains ; also in Michigan and Tennessee, and in a few other 

 States. Its thickness in Canada is estimated to be about six miles, or more 

 than 30,000 feet. About the middle of its column are the strata containing 

 the Eozoan canadense, the real nature of which is still involved in some doubt. 

 However, it appears that a majority of geologists have accepted it as the fossil 

 remains of a Rhizopod, and consider it as the commencing point of animal 

 life. 



The Huronian group has received its name from Lake Huron, in the vicinity 

 of which it is largely developed and exposed. It overlies the rocks of the 

 previous group in a manner proving its later origin. The dividing line between 

 these two Archaean groups is not sharply drawn. The absence of fossils in 

 these rocks compels us to base their distinction altogether upon their litho- 

 logical characters, which, alone, seldom affords a safe criterion for the deter- 

 mination of groups. The thickness of the Huronian group in Canada is 

 estimated to exceed 10,000 feet. None of these Archaean rocks are exposed 

 in the State of Kentucky, though their presence at great depth can not be 

 doubted. 



The rocks formed during the Palseozic age are divided into three formations : 

 the Silurian, the Devonian and the Carboniferous. 



SILTJKIAN FORMATION. This formation was first established by Sir E. T. 

 Murchison, the celebrated English geologist. He named it after the ancient 

 Silures, the former inhabitants of that portion of Britain where he first studied 

 this class of rocks. Murchison divided this formation into the Upper and 

 Lower Silurian, a subdivision generally accepted by the geologists of Europe 

 and America, and which answers fully for most purposes. The Lower Silurian 

 formation is the most important on account of its large extent in thickness, as 

 well as in area. It is subdivided into the following groups, which are given 

 in ascending order. 



1. St. John's Group; 2. Potsdam Group; 3. Calciferous Group; 4. Quebec 

 Group ; 5. Chazy Group ; 6. Birdseye Limestone ; 7. Black Eiver Group 

 8. Trenton Group ; 9. Utica Slate ; 10. Hudson River Group. Some of these 

 groups, as will be seen, are named after localities where they are prominently 

 exposed, while others have received their name from the peculiar character 

 of the rocks which they embrace. Of all these different groups, the Trenton 



OEOL. SUB. 3 



