Wadsworth.] 310 [February 20, 



less to (lark, almost black, and contains macroscopic inclusions of 

 hornblende. The hornblende is black or dark green in color, eleava- 

 ble,and fuses easily (F. 3-|-) with intumescence to a black, magnetic, 

 blebby mass. The relative abundance of these minerals is in the 

 order in which they are named, but the feldspar largely predom- 

 inates. Through the quarries, reddish and grayish fine grained 

 bands of syenite occur. jMinute and almost microscopic crystals of 

 danalite are found in some parts of the rock; and to this mineral is 

 doubtless owing the peculiar appearance of the walls in the main 

 hall of the Custom House in Boston. 



The sections of the Quincy syenite that I have examined, were 

 made by me considerably over a year ago, for comparison with the 

 Ilockport granite, but the publication of the results has been de- 

 layed in order to make further observations in the field and labora- 

 tory; finding, however, that the more important work upon which I 

 am now engaged will prevent my recurring to the subject for some 

 time to come, it seems best to present these preliminary notes and 

 leave the remainder of the work for a future time. The sections 

 examined were made principally from specimens obtained from 

 MitchelFs Quarry, Payne's Hill, Quincy, and from the stone em- 

 ployed in the construction of the east wing of Gore Hall, Harvard 

 University. 



In the thin section the hornblende is of a blue, green, or brown 

 color, and has suffered a change to chlorite in many places. Its 

 edges are very' ragged and fringed, having deep sinuses filled with 

 quartz and feldspar. Some sections show particles of hornblende, 

 extending, like boulder trains, one after the other, in wavy lines 

 through the feldspar, from points of the main hornblende mass. 

 The hornblende appears to have been first formed, and then par- 

 tially dissolved and rent asunder by the magma, before it (the mag- 

 ma) crystallized. 



The feldspar is mainly orthoclase, but here and there patches of 

 plagioclase are seen. It is much altered, and in places partially 

 replaced by quartz. The feldspar is full of inclusions (glass cavi- 

 ties, etc.), besides containing numerous fragments and microlites of 

 hornblende. The inclusions, excepting the hornblende, are generally 

 arranged in parallel bands, oblique to the line of twining. 



The quartz contains numerous fluid cavities of relatively large size 

 inclosing bubbles, whieh in the smaller cavities show the usual un- 

 ceasing motion. The microlites of hornblende in the quartz are 



