THE PETROGRAPHICAL PROVINCE OF ESSEX 

 COUNTY, MASS. IV. 



BASIC DIKES 



By far the greater part of the dikes of this region are dense 

 black rocks, evidently very basic in character. The majority of 

 these are diabases of various kinds, only a few not belonging to 

 this ever-present family and representing more unusual types. 

 However, these basic dikes have been so far but little investi- 

 gated, but it will not be amiss to describe the specimens in my 

 possession. 



Campto?iitic dikes. — Cutting the foyaite of Salem Neck and, 

 according to Sears, the " augite-syenite " of Coney Island in 

 Salem Harbor are dikes of dense, black, finely crystalline rock 

 without phenocrysts, and composed essentially of hornblende, 

 less augite, and plagioclase. These rocks are unlike typical camp- 

 tonites since there are no large and abundant ferromagnesian 

 phenocrysts, and alumina is rather high. In certain respects 

 they seem to be allied to the proterobases. They are provision- 

 ally classed with the camptonites for various reasons, among 

 which may be mentioned their connection with foyaite, certain 

 features of their chemical and mineralogical composition, and 

 their resemblance to camptonitic rocks from localities in Maine, 

 New Hampshire, Vermont, and Norway. It is, by the way, a 

 somewhat remarkable fact that no typical camptonites or mon- 

 chiquites have yet been observed in the region. 



Under the microscope these rocks are holocrystalline, and in 

 fresh specimens have a structure approaching the ophitic, though 

 the colored components are, as a rule, more automorphic than is 

 the case when this structure is typically developed. The fol- 

 lowing minerals are present : much hornblende, less pyroxene, 

 occasional olivine, plagioclase, a little orthoclase, some magne- 

 tite, and rare apatite. Neither biotite nor titanite were seen. 



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