GEOLOGY. 247 



question " was that of the venerable Dana ia December, 1888, and it may 

 easily be quoted at length : 



"1§4I-1§88."* 



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

 ucts of deposition is the definition of the (Johimbia 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, intimately 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 formation 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 upper Mis- 

 sissipiii 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 

 coi)per-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- 

 duced to a few feet of shales or limestones, and is sometimes ahuost 

 unrecognizable. It is also conspicuous in the Black flills, where, ac- 

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

 and quartzites, while the Silurian proper and the Devonian are absent, § 

 althougli tliere are no marked unconformities. Newton's observations 

 were verified and the anomalous relation of the Black Flills rocks dis- 



*Ain. Jonr. Sci., :?nl series, vol. xxxvr, i>. 427. 



t McGee, Am. Journ. Sci., 3<1 series, vol. xxxv, p. 400. 



tOp. cil. 1888, p.ige xxii. 



$ Rep. on Geology autl Kesourcos of the Black Hills, 1830, pp. 40,41. 



