226 MINERALOGY AND LITHOLOGY. 



rocks Other than basic this is known to occur constantly; but we may sup- 

 pose that when this does happen, the two rocks will have certain general 

 characters stamped upon them by the different forces that have acted 

 on them, and which will distinguish them from one another. I shall, 

 therefore, at first, point to certain fundamental differences in the micro- 

 scopic character of eruptive and metamorphic diorites, which, so far as 

 New Hampshire rocks are concerned, are general, and which therefore 

 may be regarded as confirmatory of the stratigraphical researches that 

 have been already referred to. 



In examining sections of eruptive diorites, the sharp, definite crystal- 

 lization of the hornblende is the first thing noticed. It may not assume 

 a definite form, but if irregular, the grains are compact, and possess well 

 defined outlines. As opposed to this, the hornblende of our metamor- 

 phic diorites, as seen in thin sections, is found in most diffuse and un- 

 compact forms, which are in marked contrast to those of eruptive dio- 

 rites. Instead of defined crystals and compact grains, we have fringed 

 masses, aggregates of needles, and minute, disseminated crystals, as the 

 characteristic condition. It will be shown that the agencies which have 

 been effectual in recrystallizing these rocks were gentle in their action; 

 and this microscopic distinction is what might be expected as the result 

 of a gentle metamorphism as compared with an igneous fusion. This 

 difference may be noted on comparing the sections represented in Figs. 

 3 and 4 on PI. 12, with the sections of eruptive diorites represented in Figs. 

 5 and 6 on PI. 9. There are other differences which are less funda- 

 mental, but still general in New Hampshire. The hornblende of the 

 eruptive diorites is deeply colored, appears black in the rock, is strongly 

 dichroic in sections, and gives no brilliant colors between crossed Nicol 

 prisms. The hornblende of our greenstones is not so deeply colored ; it 

 is light green in thin sections, is not so dichroic, and gives brilliant polar- 

 ization colors. In other words, the hornblende of the eruptive rocks is 

 the so-called basaltic variety; and of the metamorphic rocks it is the 

 common variety. The analysis of the hornblende of the green diorites 

 has been given on page 66. This analysis was made on quite pure mate- 

 rial, and shows that it is the aluminous variety called pargasite, a fact 

 which has been often enough shown before in studies on like rocks. 

 I would like to call attention in this connection to the analyses of horn- 



