ON A BASIC ROCK DERIVED FROM GRANITE. 671 



is the case is clearly shown by the microscopic structure of the 

 rock, as stated below. 



Microscopical details of the alteration, — A microscopical exam- 

 ination of sections illustrating all of the phases of alteration 

 shows some variation in the intermediate stages of the process, 

 but considerable uniformity in the final results. 



Sections of the least altered granite show it to consist of 

 orthoclase, microcline, and quartz, with occasional stout prisms 

 of apatite and irregular masses of tourmaline. Granite of this 

 character is quite common in the region, and has been previously 

 described by the writer. 1 The feldspar has the common dull and 

 cloudy appearance. The quartz contains abundant fluid inclusions 

 and great numbers of the hair-like bodies usually considered 

 rutile. Undulatory extinction is constant, and much granulation 

 is shown in nearly all sections. 



Alteration begins with the development of a greenish aggre- 

 gate in the feldspar. This may form irregular masses, or may 

 be confined to cracks. Very often quite large portions of the 

 aggregate have angular outlines formed by cleavage cracks of 

 the feldspar. Where the aggregate has formed in a zone of 

 crushing, it usually contains small, angular fragments of feldspar 

 which at first sight look like crystals of some newly formed 

 mineral. The areas of the aggregate are very unequally distrib- 

 uted in the granite, one portion of a specimen being greatly 

 changed, while another portion remains unaltered. In some 

 cases this is clearly due to the arrangement of cracks and 

 crushed zones, but often there is no apparent reason for it. As 

 the alteration proceeds the areas of the aggregate gradually 

 extend until they entirely replace the feldspar, leaving no trace 

 of its former presence. 



At the same time the quartz is attacked, but, as a rule, it 

 yields much more slowly than the feldspar. In the absence of 

 cleavage the alteration proceeds along the borders of the grains 

 and in the irregular cracks. The areas of alteration product 



1 C. H. Smyth, Jr.: Petrography of the Gneisses of the Town of Gouverneur, 

 N. Y. Trans. N. Y. Acad. Sci., XII., p. 210. 



