Wadsworth.] 226 [May 16, 



2. At the corner of Brighton Avenue and South Harvard St. 

 two "trap" dikes cut the " amygdaloid," changing their direction 

 considerably in the passage, the mean direction being about N. 55° E. 



BROOKLINE AND EOXBURY. 



In Brookline and Koxbury many " trap " and "greenstone" dikes 

 are to be seen. On Tremont St M between Parker and Francis 

 Streets, seven "trap" dikes and one of "greenstone" occur; three 

 of the " trap " dikes stand side by side, separated only by narrow 

 bands of conglomerate. The " greenstone " here runs about !N~. 25° 

 W., differing much in direction from the other " greenstones." In 

 Brookline, on Washington St., between Allston and Beacon Streets, 

 a "trap" dike is seen, showing an abrupt change in part of its course 

 for a short distance. 



The directions of all these intrusive rocks have been as carefully 

 ascertained as they well could be with a common pocket compass. 

 Many of them are strongly magnetic, so much so that pieces may be 

 lifted with a magnet. This strong magnetism renders the obtaining 

 of their true direction, in many cases, quite difficult. The direction 

 for the same kinds of rock varies somewhat in different parts of the 

 dikes, but in general it is the same. Their general direction to the 

 true meridian (the correction for which I am indebted to Mr. A. 

 Searle of the Phillips Observatory) is as follows: " Diorites," N. 40° 

 E.; "Traps," N. 40° E.; "Greenstones," N. 60° W.; "Porphy- 

 ry," N. 40° W. 



Special Description. 

 " Diorite." 



GRANITE STREET, OFF SOMERVILLE AVENUE, SOMERVILEE. 



This locality is one of the best and longest known in this vicinity, 

 being the famous prehnite quarry of Milk Kow, Charlestown, which 

 is the name now given in our mineralogies to this locality, Somer- 

 ville formerly having been a part of Charlestown. 



In the Danas' Geology of Boston, the rock is described as a green- 

 stone, which they define as a granular aggregate of hornblende and 

 feldspar (pages 163-201). In Cleaveland's Mineralogy the same 

 designation is applied to it. In the mineralogies of Dana, Blum and 

 Alger, it is called sienite. Dr. Webster, in the Boston Journal of 

 Philosophy and Arts, Vol. II, pages 282-286 calls this a " sienitic 







