306 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



lioniblende-porphyi-ites of the Mosquito Range unci 3 ditferecl from the 

 Mosquito rocks in containing a jjecuhar development of augite in the place 

 of hornblende.^ 



Mr. Gilbert enumerates various isolated groups of mountains in the 

 plateau region — the Sierra La Sal, Sierra Abajo, Sierra El Late, and Sierra 

 Carriso — which, from the description of geologists who have visited them, 

 he infers to be true laccolites. He also infers that their rocks are analogous 

 to those of the Henry Mountains, which is very likely to prove true in so 

 far that what he describes as porphyritic trachyte maj- correspond to the 

 porphyries with large crystals above described. His further generalization 

 that the two types of mountain structure, the laccolitic and the volcanic, 

 necessarily involve two chemical types of rock, the one acidic, the other 

 basic, is, as shown above, not authorized by the observed facts. It might 

 fairly be reasoned that the more acidic lavas, when intrusive, owing to their 

 greater viscosity, would tend to form thick, dome shaped masses like his 

 laccolites, rather than basic lavas; but even this tendency is not without its 

 exceptions. 



It is the intrusive quality, not the relative acidity or basicity of the 

 magma, to which the characteristic structure of this rock type is due. 



Dr. Peale^ has further extended the probable development of intrusive 

 bodies, more or less analogous to the laccolites in form, but furnishes no 

 decisive determination of their petrographical structure or composition 

 From specimens seen or actually collected by the writer, it may be stated, 

 however, as a fact about which there can be little question, that the type 

 of intrusive rock represented by the older series is extensively developed 

 between the North and Middle Parks, in the Middle Park, and between 

 the Middle and South Parks, that it fornis the mass of Spanish Peaks, and 

 occurs in enormous developments in the Gunnison region, where the vari- 

 eties characterized by large feldspars cut across Cretaceous strata. Similar 

 bodies also exist beneath the more recent lavas of the San Juan region, 

 wiiich lends probability to the supposed similarity of the rocks forming the 

 isolated mountains of the Sierra El Late, Sierra Carriso, and others. 



'Mr. Cross's detailed description of these rocks will be found at tLe end of Appendix A. 

 - " On a peculiar type of eruptive rocks in Colorado." Bulletins United States Geological and Geo- 

 graphical Survey, Vol. HI, pp. 551-504. 



