58 HENRY S. WASHINGTON 



In structure they are always granitic, and in texture vary from 

 rather fine to coarse-grain, the last looking like a typical 

 diorite. The only minerals visible are white feldspars and 

 black hornblende, biotite and augite. 



Under the microscope it is seen that these rocks are essentially 

 monzonitic in character, in Brogger's sense, orthoclase or an 

 alkali-feldspar being almost invariably present along with the 

 plagioclase, and that they vary from rather basic rocks rich in 

 plagioclase and poor in orthoclase to more acid ones in which 

 the orthoclase largely predominates over the plagioclase and 

 where quartz also appears. The former closely approach the 

 hyperitic varieties of the essexites, and, in fact, are only distin- 

 guished from these by their greater coarseness of grain and 

 more dioritic appearance megascopically. The latter closely 

 approach the akerites and perhaps should be described with 

 them, but, on account of their intimate association with the dio- 

 ritic rocks, and also because of their distinctly different mega- 

 scopical character, they are placed here. Between these two 

 extremes are found many transition types. The structure is 

 always granitic or hypautomorphic, the dark minerals usually, 

 but not always, having crystallized before the feldspars, the 

 plagioclase generally before the orthoclase, and the quartz, if 

 present, being always interstitial. None of the specimens are 

 quite fresh, the best in this respect being some from near 

 Collin's Cove, Salem Neck, which are hyperitic in structure. 



The plagioclase, which has a tendency to stout tabular forms, 

 is highly, and in many cases beautifully, twinned, according to 

 the albite and pericline laws. It varies considerably in compo- 

 sition from an oligoclase, Ab 3 An 1( to a basic labradorite, 

 AbjAng, the former being more abundant in the more acid 

 orthoclase-rich varieties and the latter in the more basic, espe- 

 cially in specimens from Salem Neck. It is usually xeno- 

 morphic toward the ferromagnesian minerals, but not always, 

 and is also met with as inclusions in the latter, so that it seems 

 that, although the latter began to crystallize first, during a later 

 stage the crystallization was simultaneous. Inclusions of augite, 



