2 70 The Founders of Geology LECT. 



rocks, to which he continued at first to apply the old term 

 Primary. By degrees, as he saw more evidence of parallel 

 structures in these masses, he thought that they were 

 probably altered sediments, and he referred to them as 

 Metamorphic. That portion of the series which includes 

 thick bands of limestone he proposed to consider as a 

 separate and overlying group. In the course of years, 

 working with his associates Alexander Murray and Sterry 

 Hunt, he was able to show the enormous extent of these 

 primary rocks, covering as they do several hundred 

 thousand square miles of the North American continent 

 and stretching northwards to the borders of the Arctic 

 Ocean. He proposed for these most ancient mineral 

 masses the general appellation of Laurentian, from their 

 development among the Laurentide mountains. After- 

 wards he thought it possible to subdivide them into three 

 separate groups, which he designated Upper, Middle and 

 Lower. In the course of his progress, he came upon a 

 series of hard slates and conglomerates, containing pebbles 

 and boulders of the gneiss, and evidently of more recent 

 origin, yet nowhere, so far as he could see, separable by 

 an undoubted unconformability. These rocks, being ex- 

 tensively displayed along the northern shores of Lake 

 Huron, he named Huronian. He afterwards described a 

 second series of copper-bearing rocks lying unconformably 

 on the Huronian rocks of Lake Superior. He thus recog- 

 nized the existence of at least three vast systems older 

 than the oldest fossiliferous formations. He may be said 

 to have inaugurated the detailed study of Pre- Cambrian 

 rocks. Subsequent investigation has shown the structure 

 of the regions which he explored to be even more compli- 



