lyo SIDNEY POWERS 



have come from the same underlying gneiss as the granite frag- 

 ment. The selective action of the dike-magma in including frag- 

 ments of only quartz and feldspar from the underlying gneiss is 

 comparable to the case at Mancos. This fragmentation is favor- 

 able to the contention of Day, Sosman, and Hostetter' that the 

 shattering of a siliceous rock is due principally to the expansive 

 force of quartz at the inversion point of 575°C. 



Somerville, Massachusetts: In Somerville and Medford, Massa- 

 chusetts, are a number of exposures of diabase dikes, among which 

 the Medford dike and the dikes of the Mystic River quarry carry 

 inclusions. These dikes have variable mineralogical compositions. 

 The Medford dike cuts Cambridge slate (probably of Permian age) 

 and the older igneous complex of the Middlesex Falls. It varies 

 in width up to 270 feet, and is probably over 4 miles in length. 

 The Mystic River dikes cut the Cambridge slates, and are less than 

 10 feet in width. The inclusions in these dikes are chiefly com- 

 posed of aggregates of single minerals, with vein quartz predomi- 

 nating. They are usually less than 6 inches in diameter and have 

 well-rounded surfaces. Some of the quartz inclusions are fully 2 

 feet in diameter. A list of the inclusions found in the Medford 

 dike includes^: quartz-diorite pegmatite, granite, graphic granite, 

 diorite, quartzite, quartz schist, slate, quartz, feldspar, hornblende, 

 and apatite. The inclusions in the Mystic River quarries are: 

 altered biotite granite surrounded by a secondary rim of finer 

 granite which has recrystallized with more femic constituents, 

 diorite, pegmatite, quartzite, quartz, andesine feldspar (Ab4An3), 

 hornblende, biotite, magnetite, apatite, zircon, and graphite. 



The quartz inclusions are of especial interest and have been 

 described by T. A. Jaggar, Jr.^ These fragments have an irregular, 

 clastic form. Sometimes the edges are rounded and sometimes very 

 angular, with frequent embayments from magmatic corrosion. 



^ "The Determination of Mineral and Rock Densities at High Temperatures," 

 Amer. Jour. Set. (4), XXXVII (1914), 1-39. 



2 In making out these lists the writer has looked through the collections made by 

 Professors Jaggar, Woodworth, and Palache, and is indebted to them for permission 

 to publish the results. 



3 "An Occurrence of Acid Pegmatite in Diabase," Amer. GeoL, XXI (1898), 

 203-13. 



