MUSEUM OF COMPARATIVE ZOOLOGY. it 
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 in all cases more resistance to 
decomposition than the other. 
A remarkable instance of mechanical deformation is exhibited in sec- 
tion No. 202. A large crystal of brown hornblende has been bent until 
it has the shape 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 which 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 porphyritic crystals are feldspar and 
parallel growths of augite and hornblende. 
The chlorite of No. 202 is often filled 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 
