656 The American Naturalist. [July, 
ing the valuable gems are given in detail. The greater part of the work 
is devoted to the detailed descriptions of the individual types of stones. 
In the third part of the work is given a systematic method of examin- 
ing a stone, with a key for use in the determination. A chapter is 
devoted to the means of identifying the various imitations in use in 
the trade. A list of 250 trade names of gems, with the scientific name 
of the mineral and the group in which it belongs in parallel columns, 
will prove of great value for reference—W. H. Hosss. 
PETROGRAPHY?: 
Rock Differentiation.—Harker’ contributes an interesting article 
on rock differentiation in his study of the gabbro of Carrock Fell, Eng- 
land. The hill in question consists of bedded basic lavas, gabbro, 
granophyre and diabase in the order of their intrusion. The gabbro 
is of especial interest, since it presents a simple example of rock differ- 
entiation. In its center the mass is quartziferous. Toward the periph- 
ery it passes gradually into an ordinary gabbro, and immediately upon 
the border into an aggregate composed largely of titaniferous magne- 
tite. In explaining the causes of this gradual transition in chemical 
and mineral composition, the author discards the theories usually pro- 
posed to explain similar phenomena, and concludes that, in the case 
under discussion, the separation Of the magma into its parts took place 
during the period of crystallization by concentration of the crystalliz- 
ing substances. The concentration is greatest for those minerals belong- 
ing to the earliest stages of the rock’s history, hence it is thought that 
the differentiation took place by diffusion in a fluid magma, and that 
in those parts of this magma richest in basic minerals crystallization 
first occurred. As the crystals separated, the supply of the crystallizing 
substance was kept up by diffusion from other portions of the magma 
into the basic portions. 
Another interesting feature of the gabbro mass relates to the con- 
tact effects produced by the rock in the surrounding basic lavas, some 
of which are enclosed as fragments in the midst of the gabbro. Their 
isotropic base has erystallized, and some changes have been produced 
' Edited by Dr. W. S. Bayley, ied has Waterville, Maine. 
2 Quart. Journal Geol. Soc., 1894, p. 3 
