PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 287 



are all decidedly more acid than the usual camptonites, and 

 carry higher alumina. Chemically they show marked affinity 

 with certain kersantites from Norway described by Brogger, 1 

 one of his analyses being given in IV. 



Vogesitic dikes. — A few dikes were found of a dark rock com- 

 posed of hornblende, augite, and biotite, but with alkali-feldspar 

 very largely predominating over plagioclase. A little quartz is 

 also apt to be present, which is apparently primary. These 

 rocks then have the mineralogical composition of vogesite or 

 minette, apart from the presence of quartz, and are provisionally 

 put here, since no chemical analysis has yet been made of them. 



As an example there may described a dike from Davis Neck, 

 Cape Ann, which is almost black, fine-grained, and compact, and 

 with small shining black phenocrysts of ferro-magnesian minerals. 

 Of these the pyroxene is colorless or very pale green, the horn- 

 blende of a light bluish-green, both in irregular grains, and the 

 biotite in thick plates of a light brown color and highly pleo- 

 chroic. These minerals are not distributed evenly, but occur in 

 streaks in which one or the other predominates. The interstitial 

 groundmass is of colorless granular alkali-feldspar Without 

 plagioclase or quartz. A little magnetite is present but no 

 apatite. 



Diabase. — Dikes of dense black rock, which may be grouped 

 under this heading, are very abundant. They far outnumber all 

 the other dikes put together, but as is usually the case they are 

 rather monotonous in character, as well as nearly always more or 

 less altered. Shaler's map of Cape Ann will show their abundance, 

 and to his paper 2 the reader is referred for a full discussion of 

 their occurrence, dip and strike, and other features. As regards 

 their relations to the other rocks it may be noted that they cut, 

 and are hence later than, all the other types. 



They vary from fine-grained to aphanitic, the usual change 

 in texture from center to border being often seen. In general 

 they are not as coarse-grained as the sheets, dikes, and flows of 



1 Brogger, op. cit, Vol. Ill, p. 71 

 2 Shaler, Ninth Ann. Rep. U. S. G. S., 1889. 



