180 THE CAMBRIAN. [bull. 81. 



(e) Eau Claire trilohite beds : These hold at least seven species of trilobites, of 

 which three are new, and a few brachiopods. These beds mark the lower limit of 

 calcareous matter in the formation; 450 feet. 



(/) Eau Claire grit: These layers mark the upper limit of the coarse sandstones, 

 almost conglomerates ; 680 feet. 



The mode of occurrence and character of the "Potsdam" sandstone 

 in the upper St. Croix district is described in great detail by Mr. Moses 

 Strong. 1 In this he shows the unconformity between the "Potsdam" 

 and the subjacent strata referred to the Keweenawan (Keweenaw) 

 series, and. describes the various sections of the sandstone exposed. 

 The knowledge of the local details of the distribution and character of 

 the " Potsdam" sandstone of the Mississippi region north of the Wis- 

 consin Eiver was increased by him in 1882 by the publication of many 

 sections and descriptive details that will be used in a summary of our 

 knowledge of the Cambrian rocks of that region. 2 



The " Potsdam" terrane of the lower St. Croix district was described 

 by Mr. L. C. Wooster in* the same volume with the report of Mr. Strong. 

 This report also contains considerable data that will be referred to in 

 the description of the Cambrian rocks of the region. 3 



The geographic distribution of the " Potsdam" sandstone and its re- 

 lation to the subjacent pre-Cambrian rocks and the superjacent magne- 

 sian limestone is shown with great accuracy upon the general geologi- 

 cal map of Wisconsin, published in 1881, and in the two sections at the 

 bottom of the sheet. 



A note by A. A. Young on the crystallized sands of the "Potsdam" 

 sandstone of Wisconsin describes the manner in which the original 

 grains of the sandstone are surrounded by crystalline envelopes of 

 quartz. 4 



The summary of the geology of Wisconsin by Prof. T. C. Chamber- 

 lin contains a description of the rocks of the " Potsdam " period. These 

 include the " Potsdam " epoch, the Lower Magnesian epoch, and the 

 St. Peters epoch, in part. A general description of the formations, ac- 

 companied by an account of the life of each epoch, is followed by some 

 general observations upon the Cambrian age, to which the formations 

 are referred. 5 



The occurrence of a conglomerate formed of bowlders from the trap 

 of the copper- bearing series in a matrix of sandstone that contains frag- 

 ments of Linguloid shells, apparently Lingulepis pinnaformis, is noted 

 by Mr. L. C. Wooster at a locality on the St. Croix River. From the 

 mode of occurrence of the conglomerate he concludes that it must have 



1 Geology of the upper St. Croix district. Geology of Wisconsin, snrvey of 1873-1879, 1880, vol. 3, pp. 

 390-428. 



2 [Geology of tho Mississippi region north of the Wisconsin River.] Geol. Wisconsin, survey of 

 1873-79, vol. 4, 1882, pp. 38-91. 



8 [Geology of the lower St Croix district. J Geol. Wisconsin, survey of 1873-79, vol. 4, 1882, pp. 109- 

 130. 



4 Further observations on the crystallized sands of the Potsdam sandstone of Wisconsin. Am. Jour 

 Sci., 3d ser., vol. 24, 1882, pp. 47-49. 



* General Geology. Geol. Wisconsin, survey of 1873-79, vol. 1, 1883, pp. 119-200. 



