BIOTITE TINGUAITE DYKE ROCK. 59 



syenite at Gale's point, gave this rock a special interest to 

 me, and I collected several specimens of it as addition to 

 the collection of the rocks of Essex County, in the cabi- 

 nets of the museum of the Peubody Academy of Science and 

 for special study. In October, 1896, I prepared six thin 

 sections of this rock for microscopical examination, and 

 from these sections I determined the following minerals in 

 its composition : a'girine, nepheline, sodalite, biotite, a tri- 

 clinic feldspar, microperthite, and some larger feldspars 

 that gave optical characters which led me to consider them 

 anorthoclase, as they had nearly the same structure as the 

 anorthoclase phenocrysts in the keratophyre rock from 

 Marblehead harbor.^ The black spots in the rock were 

 magnetic iron, a decomposition product of an original bi- 

 otite. The ground mass was so associated with fragments 

 :md grains of »girine, and microliths of feldspars, that it 

 was deemed necessary to have a chemical analysis made 

 of the rock l)efore determining it. In May, 1897, 1 showed 

 the specimens and thin sections of this rock to Dr. J. E. 

 Wolfi'at Harvard University, and told him my conclusions 

 as to what it was ; subsequently the specimens and sec- 

 tions of this dyke rock were placed in the hands of Dr. 

 Arthur S. Eakle of the Petrographical Laboratory at 

 Harvard University to investigate and anal3'^ze. 



Dr. Eakle has worked out a very careful and minute 

 microscopical and chemical analysis of this interesting 

 dyke rock which is as follows : — 



Macroscopically the rock has a compact holo-crystalline 

 structure, breaking with an even fracture ; and a greenish 

 orray color with a slightly greasy luster, like rock rich in 

 ue[)heline. Small phenocrysts of feldspar are scattered 

 throughout and also much magnetite in patches which 

 latter give a mottled appearance to the rock. 



» J. H. Sears, Bull. Mus. Comp. Zoology. Geographical series, Vol. No. 9, 1890. 

 ESSEX tNST. BULLETIN, VOL.]|^XXIX 5 



