12 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 



the first generation, while feldspar, augite, and biotite crystallized nearly 

 contemporaneously and form the second generation. The secondary 

 minerals are uralite, chlorite, biotite (in part]), leucoxene, kaolin (]), 

 magnetite (in part), calcite, pyrite, and quartz. The plagioclase is 

 somewhat variable in composition, owing to zonal structure, but has 

 probably an average composition corresponding to andesine. It is also 

 probable that orthoclase is present, though the potash obtained in the 

 analyses may be derived from a plagioclase containing a considerable 

 per cent, of potassium. The augite-diorite differs from the diabase in 

 that the ophitic structure is wanting, and that the brown hornblende, 

 which now comes into greater prominence than the augite, is generally 

 in idiomorphous crystals. The diorite is characterized by very perfect 

 instances of the parallel intergrowth of augite and hornblende. 



In both the diabase and diorite the change of the augite has been 

 uralitization, though in the diorite it has in many cases changed directly 

 to chlorite. 



In conclusion, I have to acknowledge obligation to my instructors, 

 Mr. J. E. Wolff, of Harvard University, and Dr. George Huntington 

 Williams, of the Johns Hopkins University. The greater part of the 

 microscopical examination in connection with this paper was made in the 

 laboratory of Mr. Wolff, and I am indebted to him for much advice and 

 suggestion. Dr. Williams has examined most of the slides, and assisted 

 me in countless ways in the preparation of this paper. I am also much 

 indebted to jSIr. R. C. Sweetser, of the Worcester Polytechnic Institute, 

 for a complete chemical analysis. 



November, 1887. 



EXPLANATIOX OF PLATE. 



Fig. 1. Illustration of the change of hexagonal plates of biotite to chlorite, with 

 separation of magnetite in concentric rims. From sections No. 202 

 and 203. 



Fig. 2. Illustration of the intergrowth of augite and brown hornblende, and 

 alteration of the former to chlorite. The alteration of ilmenite to leu- 

 coxene along the Gleifjldchen is also shown. From sections No. 202 

 and 208. 



