DISCUSSION 263 



" How by the ' eruptive process ' theory would it be possible to account 

 for the fact that the calcium sulphate may at one time be deposited as 

 anhydrite and at another as gypsum ; also that in some deposits we find 

 the potash salts and others not? " 



[Even if we regard the "bar theory" as a good explanation of such complex 

 deposits as those at Stassfurt, it is not wholly satisfactory as applied to deep de- 

 posits of pure salt, without sulfates and other chlorides. — H. L. F.] 



WILLIS T. LEE'S COMMENTS 



" In southern Colorado, extending from the mountains eastward, is a 

 series of mesas capped with lava and bearing such relations to each other 

 that they are undoubtedly remnants of a former plateau. The top of 

 this plateau has the maximum elevation of 3,500 feet above the general 

 level of the plain. The surface of the plain is traversed by numerous 

 basalt dikes, the same material as that forming the mesa caps. 



" In the vicinity of Folsom, New Mexico, south of the mesa, there 

 has been extensive volcanic activity of recent date, and one small cinder 

 cone of recent date has been found north of the mesa at Trinchera, 

 Colorado. The volume of lava issuing from the Trinchera vent was 

 very limited, and there are no dikes which are known to belong to the 

 recent period of activity. Near the northern base of the mesa occur two 

 dikes, each about three feet thick, running parallel. To all appearance 

 they are members of the system of dikes representing the conduits 

 through which the extensive mesa lavas flowed. It is possible, however, 

 that they are of more recent origin, but this I can not state positively. 

 If they belong to the older system, as seems probable, the rock now at 

 the surface was formed prior to the removal by erosion of a thickness of 

 about 3,000 feet. If they belong to the younger lavas, the configuration 

 of the surface is such as to indicate that at least a few hundred feet of 

 material have been lost from the surface since, the dikes were formed. 

 In either case a comparatively close texture would be expected for the 

 dike material. 



" On the contrary, the dike is scoriaceous and the cavities filled with 

 petroleum. The oil is of a light amber color and moderately fluid. The 

 quantity is such that when pieces of the rock are broken open the oil 

 flows out in a small stream. Large pieces, which gave no external evi- 

 dence of bearing oil, were found to be saturated throughout their mass. 

 Small pieces of the rock, heated over a Bunsen burner, lost about 12 

 per cent of their weight. 



*' The large cavities, in some cases at least, were not connected. Cav- 

 ities filled with oil were noted, surrounded by solid basalt one-fourth of 



