68 The American Geblogi.8t. January, 1894 
The chapters devoted to the characters of minerals and of rocks, 
megascopic and microscopic, and to the classification of rocks, are well 
arranged and well illustrated. The general classification of the micro- 
scopic structures of rocks is that first proposed by Zirkel, viz.: noncrys- 
talline, hemi-crystalline, glassy and clastic. The subdivisions, and 
some of the special terms are adopted from Fouque and Michel-Levy, 
and from Rosenbusch. The massive crystalline rocks are divided 
chemically into three series, viz.: acid, intermediate and basic, and con- 
densed descriptions are given of the various 6pecies belonging to each. 
Their division into two groups by Fouque and Michel-Levy based on 
two successive stages of crystallization, and into three, based on their 
internal micro-structure, viz.: granular, porphyritic and glassy, or holo- 
crystalline, crystalline and vitreous, are mentioned, but the author 
adopts the threefold division based on the relative amounts of silica. 
The schistose crystalline rocks, i. e. the metamorphic, are derivable 
both from sedimentary strata and from igneous rocks. "At one end of 
the schistose series we find rocks in which an original sedimentary 
character remains unmistakable. At the other end, after many inter- 
mediate stages, we encounter thoroughly amorphous crystalline 
masses that bear the closest resemblance to eruptive rocks into which 
they insensibly pass.'" 
The treatment of igneous rocks and of their relations to the strata 
with which they come into contact is one that shows the special quali- 
fications of the author. The igneous rocks of all ages, as seen in 
Britain, have been the object of long study by him. These and the 
crystalline schists we do not think have been so thoroughly treated in 
any other similar work. In this excellent presentation we notice but 
two points to which as American geologists we feel like taking excep- 
tions, viz.: There is no adequate allowance for internal heat of the 
earth as a cause for regional metamorphism. It seems more than 
probable that in at least Archean time the isothermal planes of min- 
eral-changing heat ran nearer the surface of the earth, and that the 
early sediments, whether of chemical or fragmental origin, and 
especially if volcanic debris was mingled with them, would receive 
through their aid largely the mineral transformations which mark now 
the Archean crystalline schists. It also seems probable that these 
planes might vary for any given region from age to age, or between two 
regions, as to their actual nearness to the surface, thus producing crys- 
talline schists at higher levels in some regions than in others. All these 
regional areas of metaniorphism the author ascribes largely to dynam- 
ical agencies (including the presence of moisture), but he also points 
out their close alliance in their metamorphic phenomena with the areas 
that can be ascribed to contact metamorphism. 
The second point to which we would take exception here is the quota- 
tion of the work of Dana and Wing on the metamorphic changes that 
can be traced out in the region of western New England between Ver- 
mont and New York city (p. 628). The author follows the old opinion 
of Prof. Dana that the schists concerned are of Trenton and Hudson 
