MUSEUM OF COMPARATIVE ZOOLOGY. 11 



in the Cretaceous formation of Silicia. The figures of the latter show a 

 sharp line of contact between the two minerals. He was also able to 

 show that, in the majority of cases at least, the minerals were in parallel 

 position. Chemical analysis showed an essential difference in the com- 

 position of the augite and hornblende. It was also observed that decom- 

 position had seldom progressed to the same point in both minerals when 

 together, though neither seemed to offer iu all cases more resistance to 

 decomposition than the other. 



A remarkable instance of mechanical deformation is exhibited in sec- 

 tion E"o. 202. A large crystal of brown hornblende has been bent until 

 it has the sliape of a letter S. The optical properties are anomalous, as 

 would be expected, and a crystal of apatite has been bent about the horn- 

 blende crystal, which has been attended with crushing, and optical dis- 

 turbances, so that the apatite crystal is extinguished in a mosaic. This 

 must be referred, however, to motions wliich existed in the partially 

 consolidated magma, as we would expect to find anomalies in the optical 

 behavior of the plagioclase grains if it were due to the action of oro- 

 graphic forces. 



Section No. 208 (corner Elm and Morrison Streets) is porphyritic, the 

 base being difficult to resolve. The porpliyritic crystals are feldspar and 

 parallel growths of augite and hornblende. 



The clilorite of No. 202 is often tilled with belonites of a green color, 

 arranged in three parallel directions, cutting each other very precisely at 

 angles of 60°. 



Summary and Conclusions. 



"What has been noted in the preceding pages may be summed up in 

 the following statements. 



The dike under consideration includes, not only the exposures of so- 

 called " diorite," but outcrops, in the vicinity of the Old Powder House 

 in Somerville, of rock intermediate in texture between the normal 

 " diorite " and normal " greenstone," as well as the " greenstone " itself. 

 The coarseness of texture is in general dependent only on the position of 

 the specimen in the dike, the fine-grained rock being naturally found 

 near the contact. The general composition of the rock is that of a 

 diabase, though facies of augite-diorite occur. 



The diabase has in general a more or less ophitic structure, and is 

 characterized by the original constituents, plagioclase, augite, biotite, apa- 

 tite, ilmenite, and magnetite ; apatite and the ore minerals comprising 



