282 GEOLOGY OF TUB DENVER BASIN". 



parallel ridges inclosing a long, narrow basin containing a stagnant pool for 

 which there is no drainage outlet. The western ridge is the larger and 

 higher. It runs with a quite regular crest, upon which are two points, one 

 of them reaching ;i height of 663 feet above Ralston ('reek at the north 

 end of the dike. Both ridges present unbroken outcrops along the top, 

 while the outer slope of each is debris covered and partially grassed over 

 Up to within a short distance of the crest. ( >n these slopes, however, shalv 

 OUtcrops appear here and there so distinctly that it is plain that the contaci 

 with the basalt is not tar from the actual outcrops of the latter. The inner 

 slopes of both ridges are also quite smooth, but basalt appeai-s occasionally, 

 down almost to the level of the basin. Mr. Eldridge found some few shaly 

 outcrops of Pierre strata on the west side of the basin, and the representa- 

 tion of the map, that this basin is caused by an included mass of Cretaceous 

 shales, has much in its favor. 



The two ridges unite at the north, and would apparently do so at the 

 south were not the point of junction covered by a terrace formation. 



At the south end of the western ridge the contact with Pierre shales 

 is very clearly seen, while the eastern passes under the drift. 



The rock of the whole mass is more compact, and hence darker, than 

 that of the Valmont dike, and is quite similer to that of the Table Mountain 

 sheets. A jointed structure producing thin, quadrangular plates or tablets, 

 from 1 to 3 inches in thickness, is well developed on both ridges. These 

 plates ring clearly when struck, and consist of quite fresh rock. A spherical 

 sundering is locally seen, and the weathering of the basalt toward the 

 southern end of the dike produces small, rudely spherical balls like those 1 

 covering portions of the surface of Table Mountain. 



The tabular structure referred to above has in part a relation to the 

 walls of the dike, which seems to the writer to he peculiar. In the lower 

 or eastern arm of the dike the tablets are formed by vertical cracks, parallel 

 and normal to the side walls, and crossed hv horizontal ones. The tablets 

 are, however, arranged at right angles to the walls of the mass, not parallel 

 to them, as would seem most natural. It would appear, therefore, that the 

 maximum contraction in this mass was in the direction of its greatest length. 

 As has been stated, this dike is situated practically parallel to the strike of 



