No. 393.] REVIEWS OF RECENT LITERATURE. 757 
clusion are briefly as follows: porphyritic intrusive masses often 
possess peripheral portions completely devoid of phenocrysts ; dykes 
connected with large intrusions may be free from phenocrysts, while 
the mass of the intrusions is filled with them ; flat crystals are often 
arranged haphazard in sheet and dykes and not in obedience to any 
law of flowage; phenocrysts often enclose crystals identical with 
those composing the matrix which surrounds them, and, finally, pheno- 
crysts are often surrounded by microlites orientated parallel to the 
bounding faces of the large crystal, indicating that the latter was 
growing after the former had crystallized. 
The conditions governing the consolidation and crystallization of 
igneous rocks are decrease in temperature, chemical composition 
of the magma, the influence of mineralizing vapors, pressure and 
increasing viscosity. In the view of the author “ the greatest deter- 
minant in the formation of rock structure is the ratio of time in the 
fall of temperature between the point where the insolubility and crys- 
tallizing moment of a compound begins to the increasing viscosity.” 
The Inwood limestone + in the northern part of Manhattan Island 
is cut by pegmatite dykes, some of which are well exposed a few 
blocks north of Fort George. Near the contact the limestone con- 
tains tremolite, biotite, and brown tourmaline, while the last-named 
mineral occurs also in the peripheral portion of the pegmatite. 
1 Eckel. Amer. Geologist, vol. xxiii, p. 122, 1899. 
