T. X. DALE — THE STOCKBRIDGE LIMESTONE. 519 



ducted. The bearing of Mr. Dale's excellent work is closely related to the results 

 detailed in the paper just read by Mr, Hobbs. Sftnilar phenomena to those de- 

 scribed by both gentlemen occur in the Cambrian gneisses at the large quarries at 

 Monson, Massachusetts, where it has been my fortune recently to discover traces 

 of a conglomeratic structure. The distortion of the pebbles consists here in a flat- 

 tening at right angles to the pressure (east to west) and a great elongation in the 

 vertical direction, with a lesser change in the third direction (north to south). 

 The tension in this latter direction expresses itself in an expansion of the blocks 

 from north to south when quarried, and which is so strong as to cause great blocks 

 to crack off from the face of the quarry under favorable circumstances with loud 

 detonations. This tension is evidently connected with mountain-making com- 

 pression. These interesting phenomena are fully described in the publications of 

 Professor Xiles. chiefly in the Proceedings of the Boston Society of .Natural History. 



The last paper was read by title: 



A CONTRIBUTION TO THE GEOLOGY OP THE GREAT PLAINS. 



BY ROBERT HAY. 



It is a fact that the study of the geology of the Plains has in times past been 

 slighted by geologists. As soon as it was possible to travel quickly to the Rocky 

 mountains, thither the naturalists of all sorts went. The upturn of the strata on 

 their flanks made it possible to study rocks of almost the whole geologic scale in 

 areas of only a few miles in extent. The neighboring Archean rocks, the faults 

 and metamorphoses, were too fascinating to leave for the slower investigations of 



the valleys of the region of the Plains. Still all who crossed the Plains made some 

 observations, and little'by little knowledge was acquired that made some general- 

 izations possible. We wish here to add some facts which, with previous knowl- 

 edge, will possibly justify a few other generalizations. 



From the southern slope of the Black hills, in Dakota, to the Panhandle of Texas, 

 and from the lOllth to the lollh meridian, the surface terrane of the Plains on the 

 level interfluvial spaces is a fawn-colored calcareous and arenaceous clay, which is 

 of late Tertiary age in its oldest parts and probably shades into post-Pleistocene on 

 its eastern boundaries. It includesthe Equus beds of Cope, but usually is barren of 

 fossils. It varies from 3 or 4 feet to 200 feel thick. It isthinnedoff by Quaternary 

 erosion on the slopes of the valleys. '1 "his erosion has also leached out in many 

 places all its calcareous and argillaceous ingredients, and left its sand to be piled 

 into eolian dunes. The bottom of this Plains marl rests on a much eroded surface, 

 which is mostly formed of another Tertiary formation, hut in places the immedi- 

 ately subjacent mck is some ( Iretaceous terrane. 



This Tertiary formation under the marl is. in the northern part, the White River 

 beds, which in Pine ridge attain a thickness of 7011 or 800 feet. South of the 10th 



parallel and east of the 103d meridian this gives place to the Loup Fork, whicli 

 rests on the Cretaceous without the intervention of the White River beds, and 

 which is characteristically developed toward the northeast in Nebraska. In this 

 region, from about the 1 1st parallel to the 35th, the Loup Pork has a \ arying thick- 

 ness of from a very few feet or a mere trace to nearly 100 feet. These thicknesses 



are those found in outcrops in the valleys of lern erosion. We cannot he certain 



of it elsewhere; we can only infer approximately. It is the main water-bearing 



