50 



WISCON'SIISr ACADEMY SCIEKCES, ARTS, AND LETTERS. 



The following analyses, made by rae for tlie State Geological Sur- 

 vey, show at a glance, the change in chemical composition which 

 has taken place in the process of the formation of .the sandstones 

 from the Cupriferous rocks: 



No. 1. (239, of the survey collection) is a fine-grained greenish 

 gray diabase, from the Fond du Lac copper mine, Douglas count}'. 

 It contains a trace of metallic copper. No. 2, (lO of the collection), 

 is brownish-black melaph3're from the Ashland copper mine, near 

 the mouth of Tyler's Fork, Ashland county. No. 3, (44) is coarse 

 grained, reddish. Bad River sandstone, from Lehighs, Bad River. 

 Crystals of feldspar can easily be distinguished in the specimen. 

 No. 4. Typical Lake Superior sandstone from a large quarry on 

 Basswood Island, Lake Superior. It is extensively used as a build- 

 ing stone. The material for the walls of the Milwaukee court- 

 house was obtained from this quarry. 



5. Potsdam sandstone. — At St. Croix Falls, Potsdam sandstones, 

 characteristic of the light-colored Primordial sandstones of the 

 Mississippi valley, come in contact with the Cupriferous rocks at 

 numerous localities. At the Falls they are usually fine-grained, 

 and are also of ten aluminous and somewhat shaly. The shaly 

 beds are often highly fossiliferous. Dr. Owen states that at St. 

 Croix Falls, " the oldest Palaeozoic fossils of this continent, if not 

 of the world, are found." At the western end of the old St. Croix 

 dam about one half mile above the Aullage of Taylor's Fails any 

 number of shaly slabs may be obtained almost entirely made up of 



