GEOLOGICAL AND MINEEALOGICAL NOTES. 115 



ceedingly tough. Two forms of porphyritic crystals are 

 seen, one glassy, long, lath-shaped and the other dull, 

 white and hexagonal. 



III. THE MICROSCOPICAL STRUCTURE. 



Thin sections of the phonolite dyke rock, when studied 

 under the microscope in polarized light, show that it is 

 composed of some crystals of sodalite, hexagonal in out- 

 line, and numerous long irregular feldspar phenocrysts 

 which are sometimes in Carlsbad twins with a quite fine 

 multiple twinning and in one section the double twinning 

 of the microline structure. Several of the feldspar crys- 

 tals have a perfect, square, cross-section which is very 

 noticeable and suggests a resemblance to the anorthoclase 

 phenocrysts which were described in my paper on kerato- 

 phyre 1 from Marblehead Neck. Micro-chemical tests of 

 this feldspar in hydro-fluosilicic acid give, upon evapora- 

 tion of the acid, equal numbers of crystals of sodium 

 (Na 2 O) and potassium (K 2 O), but with no calcium 

 (Ca O) ; sp. gr. 2. 572 'to 2.58. The analysis of the 

 ajiorthoclase feldspars in the keratophyre rock which was 

 made at the laboratory of the U. S. Geol. Stirv. at Wash- 

 ington by Dr. Thomas Chatard gives K 2 O, 6.98 ; N% O, 

 6.56. This micro-chemical test, therefore, shows that the 

 feldspar in this phonolite rock is very near if not chemi- 

 cally equal to anorthoclase. The hexagonal outlines of 

 the sodalite phenocrysts are isotropic and the mineral 

 gelatinizes readily with acid which upon evaporation gives 

 an abundance of common salt crystals. There are also 

 some crystals of green augite and brown hornblende, one 

 of the outline hornblende crystals being filled with minute 

 crystals of eegiriue. The holo-crystalline ground mass is 



5 * Bulletin of Museum of Comparative Zoology at Harvard College, Geological 

 Series, Vol. u, June, 1890. 



