ERUPTIVE EOGKS. 305 



as the t}-i)e of the rock, it would be classed as plagioclastic, while in the 

 rock as a whole, or in those varieties in which the large orthoclases have 

 not been developed, orthoclase predominates. It is apparent, moreover, 

 that, owing to the increased facilities which the microscope now affords for 

 the detection of plagioclase among the microscopical constituents of a rock, 

 an ever-increasing number of rocks hitherto supposed to be orthoclastic 

 will be found to have a predominance of plagioclase feldspars, and that, if 

 this distinction remains without modification as a basis of classification, the 

 extent of rock species of the orthoclastic type will become more and more 

 restricted and eventually rather rare/ 



Distribution of intrusive rocks in the Rocky Mountains. ^Phe older and intrUsive 



series of rocks, represented in this region by the porphyries, porphja-ites, 

 and diorites, form undoubtedly a very large proportion of the igneous 

 rocks of Colorado and adjoining regions which have hitherto been classed 

 as Tertiary eruptives or as eruptive granites. To how great an extent they 

 should be substituted for the latter on the existing geological maps it is not 

 yet possible to determine with accuracy, owing to the incompleteness or 

 absence of characteristic specimens. An opportunity was, however, offered 

 in the case of the Henry Mountains, so ably described by Mr. Gilbert, who 

 kindly loaned a considerable number of the actual rock specimens and 

 sections, upon which the determinations for his work were founded. These 

 were submitted to Mr. Cross for microscopical examination, several new thin 

 sections being made by him for this purpose. The results of his investiga- 

 tion, although (owing to the incompleteness of the series and the altered 

 condition of many of the specimens) not adequate to afford a complete 

 characterization of all the rock masses found there, show conclusively that 

 they belong to the same structural type as the older intrusive rocks of this 

 region. Out of 19 varieties represented by specimens or thin sections, 14 

 were found to correspond very closely in composition and structure to the 



'A remarkable instance of this tendency is found in the recent review of rock determinations of tLe 

 fortieth parallel by Messrs. Hague and Iddings (American Journal of Science, ssvii, 453, 1884), which 

 shows that in the vast area covered by that survey only a single true trachyte, and that not of the 

 most characteristic type, was observed, although in the original determinations, made in the light of 

 the host petrograpbical science as it existed twelve years ago, these rooks were supposed to form a 

 largo and important class there. 

 MON XII 20 



