244 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



produce a change in the chemical composition of the rocks. Upon ex- 

 amination it will be found that such a change can be observed in many 

 of the dikes. It stands in direct proportion, so far as known, with the 

 chemical composition of the strata traversed. Dr. Peale and Mr. Holmes 

 have collected specimens from the Western groups of porphyritic tra- 

 chytes that defy identification by a lithologist. It was found in such 

 instances, upon partial examination only, that the rock contained con- 

 siderable quantities of the main constituents shown by such strata 

 as it had passed through. Thus, for instance, one specimen from a 

 dike contained carbonate of lime. Investigations being made, it was 

 found to penetrate a series of calcareous shales. It is evident that the 

 calcium could not at first have been assimilated as a carbonate. It was 

 taken up as a carbonate, turned by heat into calcium oxide, and remained 

 as such until percolating waters and exposure to atmosphere enabled 

 it to acquire a sufficient amount of carbonic-acid gas again to form 

 carbonate of lime. Many similar instances have been observed, show- 

 ing how widely spread this assimilation is. It remains to be stated that 

 no ultimate chemical examinations have been made of these cases as yet, 

 or more definite results would be presented. 



Clearly the opportunity for assimilating such foreign material is 

 greatest in a narrow fissure. In proportion the volcanic lava has ex- 

 posed to its action a far greater surface of unchanged rock. Its narrow 

 width will more readily permit a thorough permeation of the entire mass, 

 and its peculiar methods of cooling may be productive of more striking 

 results. In dikes, therefore, rather than in the large masses of yoI- 

 canics, must we look for the variations produced by the foreign matter 

 entering into the composition of the erupted material. 



As the result of additional quantities of silica, alumnia, lime, mag- 

 nesia, and other constituents, from the source above described, totally 

 changes the original character of the lava, I comprise the series under 

 the name of " symmorphic" rocks. 



VI.— CORRELATIOX WITH OTHER ERUPTIVES. 



Viewed in their character as eruptives, the porphyritic trachytes may 

 be compared to the granites of the Elk Mountain region. Both are 

 isolated, both have broken through sedimentary strata — probably at or 

 after the close of the Cretaceous period. Both classes of occurrences are 

 accompanied by disturbances of the sedimentary beds. Dr. Peale* has 

 described them both in the same i^aper, and points out their analogies 

 and dissimilarities. He has seen the most typical points of either class, 

 and is competent to judge. 



So far as I can determine, there is no direct correlation between the 

 eruptions of the group under consideration and the great, massive 

 eruptions of the Uncompahgre, San Juan, or other regions. I consider 

 it highly jjrobable that there is a chronological and dynamical con- 

 nection, but none that appears as such at the present time. From all 

 that can be learned the eruptions of porphyritic trachytes occurred very 

 near the time that we must assunje for those of rhyolite, prior to basalt. 

 I am unable to state which of the two, rhyolite or porphyritic trachyte, 

 should be regarded as the older. In mineralogical constitution the 

 rocks of the latter group have a general resemblance to trachyte proper j 

 less so to rhyolite. The lower percentage of silica, and its usual 

 absence as such, constitute a considerable portion of the similarity. 



