176 II S. Washington — Solvsbergite and Tinguaite. 



Art. XYII. — Solvsbergite and Tinguaite from Essex County, 

 Mass.; by Henry S. Washington. 



A visit to the Christiania Region made last summer, in 

 company with other American and English geologists, under 

 the guidance of Prof. W. C. Brogger, greatly added to an 

 interest in the closely parallel region of igneous rocks of Essex 

 Co., Massachusetts, which had been already awakened through 

 several visits. A series of microscopical and chemical exami- 

 nations was undertaken, with the aim of comparing the two 

 regions. In the course of this several rocks were shown to be 

 types of such peculiar interest that it was decided to publish 

 their descriptions in advance of a more general paper on the 

 subject. 



Glaucophane-Solvsbergite. — The only mention of rocks be- 

 longing here, which I can find, consists of brief notes by 

 Shaler* based on observations by R. S. Tarr, who describes 

 dikes 3, 182 and 184 as quartz and feldspar porphyries con- 

 taining glaucophane in the groundmass. Rocks from all 

 these have been collected, but the specimens most thoroughly 

 examined came from dike 184, — or rather from JSTo. 184a of his 

 list of dikes (p. 593). 



The dike occurs at Andrew's Point, the northeastern ex- 

 tremity of Cape Ann, cutting the granite and running into the 

 sea. It averages about 4 feet in thickness, with a strike of 

 N*. 45° W. (Shaler) and vertical dip. The structure is schistose 

 parallel to the walls (well brought out by weathering), and it 

 is split by joint-planes almost at right angles to its length. 

 These joint-planes coincide with a secondary system of planes 

 cracking the granite on either side, in most cases passing unin- 

 terruptedly from one into the other, and it is evident that both 

 were acted on by the forces producing these simultaneously. 

 There is no evidence of contact action on the granite, but the 

 dike rock becomes notably coarser-grained toward the center, 

 though still remaining fine-grained. The dike rock also carries 

 two or three small inclusions of the wall material, and sends 

 off a few slender apophyses. There can be absolutely no doubt 

 that it is a true dike of igneous origin. 



The rock is rather dark, slightly bluish gray, finegrained 

 and compact, and splits quite readily parallel to the walls. No 

 phenocrysts are visible. The specific gravity is 2*703 at 22° C. 



Under the microscope the rock shows a holocrystalline, 

 microgranitic structure. True flow structure is not marked, 

 but the schistose character is rendered evident by long streaks 



* Shaler, Geology of Cape Ann, 9th Ann. Rep. U. S. G. S., 609, 1889. 



