IGNEOUS FOEMATIONS. 283 



the inclosing shales: and it is a demonstrated fact that the conductivity 

 of sedimentary rocks is far greater parallel to their stratification than normal 

 to it, yet ii seems remarkable that this difference should have been felt 

 throughout a mass of liasalt 1 -mile in length l>\ 350 yards in width. This 

 tabular sundering is perhaps best developed near the center of the eastern 

 dike, opposite the basin. In this connection it is to be noted that the 

 included Pierre shales must have become very highly heated and, as a 

 consequence of their position, could not cool faster than the surrounding 

 basalt. In further explanation of this unusual structure, it may be suggested 

 that the continued passage of molten lava through this channel so heated 



the upturned shales on both sides that the greater conductivity parallel to the 

 strike may have been able to manifest itself throughout the mass. 



The large arm of this dike exhibits upon the crest near the northern 

 end a pronounced tabular structure, in which the plates stand nearly vertical 

 and are arranged in a gentle cun e with the convex side to the north, i. e.. the 

 plates are approximately parallel to the probable end of the outline of the dike. 

 The above-mentioned law of conductivity in sedimentary rocks no doubt 

 explains this local structure. Tabular sundering is, however, not marked 

 in the greater part of this arm, the joints producing no regular structure. 



HILLS BETWEEN THE RALSTON DIKE AND TABLE MOUNTAIN. 



Between the Ralston dike anil Van Bibber ('reek' are a number of 

 small knolls or hills representing irregular arms of basalt which penetrate 

 Cretaceous strata. These outcrops all lie nearly southeast from the south 

 end of the Ralston dike, and in the direction of Table .Mountain. 



Beginning near the Ralston dike, the first basalt outcrop is in a round 

 knoll. Only the top shows basalt in place, and shaly strata outcrop on all 

 sides not far below. About 200 feet west of this is a short dike of irregular 

 outcrop. 



Between Ton and son feel southeast from the above knoll is a very 

 irregular cone L50 feet high. This is caused by a dike which cuts through 

 the apex of the cone, approximately parallel to the strike of the strata, but 



1 The local Dame of tins small water course is Dry Creek, but as there are several others of the 

 same name within the limits of this map, the name used upon the Hayden map is retained. 



