ENDUCH.] ORIGIN OF COLORADO ERUPTIVES. 271 



It is to be regretted that not more analyses of other volcanic rocks 

 Avere available, or we would probably be able to see the result ; the 

 Trachyte does not contain the lowest average of silica of the trachorheitic 

 group. 



If we assume that the normal composition of a given metamorphic 

 granite were identical with that of the trachyte, containing 60.87 per 

 cent, of silica and 3.76 of ferrous oxide, and we subject that granite to a 

 process of fusion and recrystallization, what would be the result ? If 

 we could produce such conditions that only a portion of the mass were 

 to be fused at a time, and the entire quantity to be used up succesvsively, 

 we would probably find that those portions requiring the smaller 

 amount of heat or the least powerful solvent agents would be separated 

 first. This result would furnish us with the mostreadily-melting product, 

 with the one containing the least silica and most iron. Repeating the 

 process upon the same block with or without the admixture of new 

 material of identical composition, we would obtain higher percent- 

 ages of silica, smaller ones of ferric compounds. Such a process could 

 be continued until silica no longer found sufQcient quantities of other 

 oxides to form silicates, and would make its appearance in the product 

 of fusion as free quartz. Conditions analogous to this I assume to have 

 occurred at the time of the genesis of the trachorheites. 



From all evidence, the eruptions of the Elk Mountain granites were 

 attended with greater demonstrations of violence than those of the 

 trachorheitic areas. In that instance we may have a more complete, 

 more rapid fusion of the original mass, and, in consequence, a crystalline 

 aggregate upon cooling. How much effect the various methods of cool- 

 ing must have had upon the lithological and mineralogical character of 

 the molten material has been suggested in previous pages. 



From the uniform character of the trachorheites it may be inferred 

 that they have had their origin at comparatively shallow depths, within 

 zones of similar rocks. Little or no violence attended their ejection, in 

 conformity with the rule as i^ronounced by Ootta,* that the erupted 

 masses rarely produce extensive mechanical or chemical changes. The 

 ejection of lava has taken place through fissures. Although evidently 

 in a highly viscous state, as a rule, it flowed for long distances, quietly 

 leaving the opening through which it was brought to the surface. 



Dissimilar in certain respects are the eruptions of porphyritic tra- 

 chytes. Greater demonstrations of force are noticeable within their 

 range. Either one or more reservoirs furnished the material for a group 

 or cluster of groups. Their singular coincidence of-structure argues 

 for a common origin, and for identical mode of ejection. Breaking 

 through the same beds at different localities, the lavas have met with 

 the same obstacles, and, obeying the same forces, have produced the 

 same results. 



The basal toid group represents types that I regard as having under- 

 gone more than one fusion. They contain a low percentage of silica 

 and a high one of ferrous oxide. Although occurring isolated, the 

 largest massive eruptions observed were in connection with other erup- 

 tions. I am aware that this is not always the case; but where it is not, 

 I should regard the basaltoids as generally relatively younger. Excep- 

 tions to this rule are not wanting, however. Inasmuch as these rocks 

 essentially form a transition to the lavas of the present day, their per- 

 sistence as frequently fused masses receives additional proof. We can 

 scarcely assume that the periodical ejections of lava, which we now 

 observe, should always indicate the alteration of fresh material. We 



* Geologie der Gegenwart. Leipzig, 187::^, p. 128. 



