232 GEOLOGY. 



CONTACT PHENOMENA. 



But the determination of the mineral composition of rocks is not the 

 highest end of microscopic work. The mere enumeration of the comjjo- 

 neuts of various stones can jtrove wearisome in the extreme. When 

 the microscope comes to aid in the solution of geological problems, then 

 its developments become of the greatest interest. It has been ai)plied 

 with greatest success in the study of the changes induced on schists and 

 stratihed rocivs by the influence of eruptive masses. The study of these 

 so-called contact phenomena had been pursued by students such as 

 Daubree and Lossen, in the field, with most satisfactory results, but it 

 never became so interesting as when Eosenbusch applied the microscope 

 and chemical analysis to the study of the rocks that lay against the gran- 

 ites in the Vosges. 



Another remarkable case of contact has during the past year been 

 studied and described by myself. In the celebrated Crawford Kotch of 

 the White Mountains an immense mass of granite, many square miles 

 in extent, comes in contact with an area of hydrous schists. If one ap- 

 proacl^es this contact from the side of the granite, most marked changes 

 are seen to take place, and changes of such a nature that the granite 

 would not be recognized as such in the neighborhood of the contact. In 

 fact, it becomes a quartz porphyry. But the microscope and analysis 

 prove that it is the same granite only modified in the style of its crys- 

 talization by the quicker cooling induced by the cold walls. On the side 

 of the schist, however, not merely have the rocks been physically altered 

 by the heat of the granite, but they have been materiall^^ altered in com- 

 j)osition, and filled with new minerals by reason of an impregnation with 

 vapors and solutions brought up and forced far into them by the eruption 

 of the granite. The granite upon the contact is mixed with fragments 

 of various kinds of schist which were torn from the walls as the molten 

 rock ascended, and the granite has in places thrown long arms out into 

 the schist. There are seven distinct zones marked by different minerals 

 and structural features between the granite and schist, and these zones 

 have definite boundaries and owe their origin to the contact. This 

 same granite in meeting the schists at another j^oint has induced the 

 same series of zones, but the zone of mixed schists and granite is moun- 

 tain great, forming, in fact, Kearsarge and Moat Mountains. I speak of 

 this study as a stage of progress because contact phenomena have re- 

 ceived little attention in America, and the features that determine the 

 eruptive nature of a granite have been ill defined. Granite means al- 

 most anything in this country, and most of the granite masses, including 

 tbis one, have by some been classed among metamorphic rocks. Internal 

 structure and lines of stratification, which granites frequently present 

 have been considered as evidences of sedimentary and metamorphic ori- 

 gin, although it is well known that volcanic products are often stratified. 



