90 Prescotfs Sketch of the Geology and Mineralogy 



SPECIES 20. HORNBLENDE AND HORNBLENDE ROCK. 



Hornblende, is diffused in great abundance, forming essential 

 ingredients in greenstone and sienite. It occurs crystallized, 

 foliated and compact, at Nahant, Lynn, Salem, Marblehead, and 

 Beverly. The principal localities of crystallized hornblende 

 are, the eastern part of Nahant, near Hospital Point, Beverly, 

 and Marblehead. The most celebrated localities of the foliated 

 variety occur at the eastern part of Nahant, and Salem Neck. 

 In the latter place it is associated with mica. Foliated horn- 

 blende occurs also in numerous bowlders of hornblende rock, 

 which are found scattered in great profusion in almost every part 

 of Danvers. These hornblende rocks, are more or less im- 

 pregnated with chlorite, which imparts to them a greenish color; 

 the chief of the hornblende is of a jet black, some however has 

 a tinge of green. 



A bowlder of hornblende rock has been found in Danvers, 

 containing columns or needles' of black hornblende, from half an 

 inch to two inches in diameter, and several inches or a foot in 

 length, crossing each other in various directions, and which, 

 contrasted with the quartz and white feldspar, of which the base 

 of the rock is composed, give it a picturesque appearance, not 

 unlike rocks that are traversed in like manner by crystals of 

 schorl. Bowlders of an indistinct variety of hornblende slate 

 occur at Nahant. 



SPECIES 21. CHLORITE AND CHLORITE SLATE. 



Chlorite and chlorite slate exists in the northern and eastern 

 sections of Lynnfield, and the western and north western parts 

 of Danvers. It exists in great abundance in Reading, and part 

 of Middleton, from whence great numbers of bowlders have 

 been carried southwardly into those towns lying in that direc- 

 tion ; some of this chlorite is of the earthy variety, but mostly of 

 the foliated. It is generally of that peculiar green color so com- 

 mon to that mineral, but in Danvers, where it approaches the 

 greenstone, (and is associated with hornblende) it becomes 

 darker, and at last nearly or quite black. Much of the green- 

 stone in that section is impregnated with it. 



Chlorite occurs at Crystal beach, Nahant, and is associated 

 with feldspar, forming a species of sienite at Little Nahant, 



