GEOLOGY. 171 



of Lake Superior, and declares that the facts now established 

 in that region show (1) the existence of an ancient gneissic 

 and granitic system, regarded as Laurentian, which is over- 

 laid unconformably by (2) a series of quartzites, schists, and 

 diorites, with limestones and some gneiss and granites, refer- 

 red to the Huronian. This is followed, probably unconform- 

 ably, by (3) the copper-bearing series (Keweenian), which 

 includes greenstones and melaphyres, and also great thick- 

 nesses of interstratified sandstones, shales, and amygdaloids, 

 the whole havingr a thickness of several miles. These are 

 finally covered unconformably by a series of unaltered hori- 

 zontal sandstones, including numerous fossils related to those 

 of the Potsdam sandstone. Irving observes : " As to any of 

 the Wisconsin or Michigan crystalline rocks being altered 

 equivalents of the Primordial and newer strata of the Eastern 

 States, such an hypothesis is certainly untenable for a mo- 

 ment." And, while admitting that such things may be else- 

 where, adds: "There certainly has been no period of meta- 

 morphism in the region of the Xorthwestern States since the 

 beginning of the Primordial." Mr. Sweet, who has particu- 

 larly studied the rocks of this region, finds the thickness of 

 the Huronian, so far as observed, about 5000 feet. The 

 Keweenian, which extends in the form of a great synclinal 

 from Lake Superior westward across the whole State of Wis- 

 consin into Minnesota (a length of over 300 miles, and from 

 thirty to fifty miles wide), has a thickness of strata estimat- 

 ed at not less than 40,000 or 50,000 feet. The trappean 

 rocks of this series, as seen at the Falls of the St. Croix, 

 were, by Owen, regarded as more recent than the Potsdam 

 sandstone. It is, however, shown by Sweet that the trap- 

 pean rocks in question not only belong to a stratified series, 

 but that the basal beds of the sandstone resting upon them 

 are in part made up of the ruins of these rocks. These sand- 

 stones have yielded several species of Conocephalites, Agno- 

 stus, and Dikellocephalus, besides Obolella and Lingulepis. 



EOZOIC ROCKS WEST OF THE MISSOURI. 



Hunt has published some preliminary observations on the 

 crystalline rocks of certain parts of the Rocky Mountains. 

 The o-neisses in the Colorado ran^e as seen in Clear Creek 

 canon, and in the Sangre de Cristo range, near Garland, 



