GEOLOGY. 247 



question " was that of tbo venerable Dana in December, 1888, and it may 

 easily be (Rioted at length : 



"1841-1888."* 



An event of some importance to students of the multifarious prod- 

 ucts of deposition is the deliiiition of the ('olumbia formation. There 

 is a break in geologic history, as commonly interpreted, between the 

 Neocene and the Pleistocene, — a hiatus partly natural and partly taxo- 

 nomic, and exceedingly difficult to close by reason of diverse methods 

 of classification as well as by reason of the dearth of common phenom- 

 ena. But the formation under consideration is a superficial deposit of 

 known genesis, intinuitely connected with the other Pleistocene depos- 

 its of the country ; it is at the same time a Ibssiliferous sedimentary de- 

 posit as intimately connected with the Neocene and Eocene formations 

 of the middle Atlantic slope as these are connected among themselves; 

 and thus the formatiou not only covers the natural discontinuity be- 

 tween the Neocene and Pleistocene, but, since it is susceptible of classi- 

 fication with either, closes the taxonomic hiatus as well.t 



There has long been grave uncertainty as to the relations among cer- 

 tain members of that Silurian or Cambrian rock series of the upjier Mis- 

 sissippi Valley known in part as the Lower Magnesian Limestone of 

 Owen ; and it is a source of gratification to geologists to note that, in 

 the second volume of his final report, N. H. Winchell has "clearly set 

 forth the relations of the various members of this series, particularly in 

 Minnesota and Wisconsin. The series, beginning at the base of the 

 well characterized St. Peter Sandstone, is as follows: (1) Shakopee 

 Limestone; (2) New Richmond Sandstone; (3) main body of limestone, 

 ("Lower Magnesian" in part) ; (4) Jordan Sandstone; (5) St. Lawrence 

 Limestone; (6) a bed of shales; (7) Dresbach Sandstone (the last four 

 representing the St. Croix); (8) a bed of shales; and (0) Hinckley 

 Sandstone (the last six constituting the Potsdam of Wisconsin); the 

 whole resting upon the red shales and sandstones which pass into the 

 copi)er-bearing series. | 



One of the most puzzling problems which the geologists employed in 

 the western Territories have been called upon to solve is the absence 

 of formations elsewhere of great volume, without marked unconformity 

 between the older and newer deposits. This condition is especially con- 

 spicuous in the Rocky Mountains, where the Silurian is sometimes re- 

 du{!('d to a fi^w feet of shales or limestones, and is sometinies almost 

 unrecognizable. It is also conspiciu)us in the Black Hills, where, ac- 

 cording to Newton, the Cambrian includes but 250 feet of sandstones 

 and quartzites, while the Silurian ])roper and the Devonian are absent, § 

 although there are no marked unconformities. Newton's observations 

 were verified and the anomalous relation of the Iila(;k Hills rocks dis- 



*Ain. Jour. Sci., :}r<l series, vol. xxxvi, p. 427. 



t McGee, Am. Jonrn. Sci., 3(1 series, vol. xxxv, p. 4GG. 



tOj). eit. 1888, page xxii. 



$ Kop. ou Geology ami Kosoiircoa of the Blaclt IHIIh, 1880, pp. 40,41. 



