GEOLOGICAL AND MINERALOGICAL 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 stiidied 

 under the microscope in polarized light, show that it is 

 composed of some crystals of sodalite, hexagonal in out- 

 line, and numerous long irregulär feldspar phenocrysts 

 Tvhich are sometimes in Carlsbad twins with a quite fine 

 multiple twinning and in one section the double tvvinning 

 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^ 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 

 (Nag O) and potassium (Kg^)? ^ut with no calcium 

 (Ca O) ; sp. gr. 2.572 to 2.58. The analysis of the 

 anorthoclase feldspars in the keratophyre rock which was 

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

 ington by Dr. Thomas Chatard gives Kg O, 6.98 ; Nag O, 

 6.56. This micro-chemical test, therefore, shovvs 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 Isotropie and the mineral 

 gelatinizes readily with acid which upon evaporation gives 

 an abundance of common salt crystals. There are also 

 8ome crystals of green augite and brown hornblende, one 

 of the outline hornblende crystals being iilled with minute 

 crystals of segirine. The holo-cry stalline ground mass is 



l 1 Bulletin of Museum of Comparative Zoölogy at Harvard College, Geological 

 Series, Vol. ii, June, IbiJO. 



