PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 1 1 1 



defined and more uncertain in outline. The structure as a whole 

 is a rather peculiar one. The rock would seem to be not very- 

 acid and with rather high potash, and its occurrence in connec- 

 tion with the gabbro (which it will be remembered also carries 

 some orthoclase) is noteworthy. 



A rock which is essentially an alkali-sye?iite- porphyry occurs 

 as a dike on the southeast coast of Marblehead Neck cutting the 

 rhyolite. It somewhat resembles a minette, though rather more 

 acid. Hornblende is absent, diopside rare, and small stout 

 crystals of biotite abundant. These are usually a peculiar light 

 brown and pleochroic, but some are seen of a pale green, 

 slightly bluish in tone, and with feeble pleochroism. This variety 

 also occurs intergrown with the brown, and may be a bleached 

 form of the latter, but it resembles closely the similarly colored 

 biotite found in some of the granites, especially that of Marble- 

 head Neck itself. The groundmass of alkali-feldspar is granular, 

 much of it being decomposed with formation of kaolin. A few 

 grains of probably secondary quartz are present. 



PA1SANITE-SOLVSBERGITE-TINGUAITE SERIES 



A small but very interesting group of dike rocks is distin- 

 guished mineralogically by the combination of alkali-feldspar 

 and either glaucophane-riebeckite or aegirite. These carry 

 abundant quartz at the acid end of the series (paisanite), little 

 or no quartz in the intermediate members (solvsbergite), and 

 nepheline in the most basic (tinguaite). This series, it will be 

 observed, corresponds very closely to Brogger's Grorudite- 

 Solvsbergite-Tinguaite series from the Christiania region. 1 As 

 far as my observations go, these dikes are not very wide, a few 

 feet at the most. Nearly all my specimens are from dikes cut- 

 ting granite, either on Cape Ann or along the Manchester-Mag- 

 nolia shore ; one specimen only is from the foyaite area of 

 Salem Harbor. 



Paisanite. — The only example of this rock which I found is 

 Shaler's dike No. 3, at the extreme southeast corner of Magnolia 



1 Brogger, op. cit., I. 



