14 COLORADO COLLEGE STUDIES. 



above the main beds of rock-salt, are the only vestiges of 

 plant-life known by the writer to have been found in the 

 Geuda measures. 



In the limestones of the Geuda, geodes of celestite are 

 frequent. 



Traced by the prospector's drill-records away from its 

 outcrop to its deepest known extensions, the Geuda grad- 

 ually changes its composition, the variegated shales and 

 the gypsum and limestone gradually disappearing and their 

 place being taken at first partly and at length wholly by 

 more decidedly sahferous shales of almost constantly blue- 

 gray to slate-color and massive beds of more or less nearly 

 pure rock-salt, which constitute the "salt-measures" of cur- 

 rent parlance. It is rot positively known that these salt- 

 measures everywhere come, in their entire thickness, with- 

 in limits that correspond with those of the Geuda outcrop; 

 but their dip and position with reference to higher and lower 

 formations are such as to indicate their substantial equiva- 

 lency to part or (as in the case of Anthony, at least) to 

 practically all of that outcrop. The gradual disappearance of 

 limestones and gypsum with depth, implies a progressive 

 change of physico-geographic conditions in the region be- 

 tween the outcrop and the deep parts of the Geuda, showing 

 that the leaching out of the salt in and near the outcrop — to 

 the probability of which Prof. Hay has called attention in his 

 "Geoloirv of Kansas Salt" and elsewhere — is not the sole 

 (though doubtless a partial) cause of the comparative ab- 

 sence of salt from the Geuda outcrop, and that this absence 

 is partly due to local differences in the physical conditions 

 under which the Geuda sediments were originally laid down. 



The salt is present in every grade of occurrence from 

 that of minute particles impregnating the shale, through 

 that of rock-salt and shale intermingled in about equal pro- 

 portion, to that of missive beds in which cla}' appears onl}' 

 as sparsely scattered flecks. As may be inferred from 

 what has been already said, the proportion of salt in the 



