REVIEWS 87 
Following the descriptions of these rocks throughout the Appalachian 
region, the distribution and petrographic characters are given in detail 
for western North Carolina. Sixty photomicrographs illustrate the mineral- 
ogic and structural varieties and modes of alteration of the rocks described, 
and their chemical relations are shown by Hobbs-Broégger diagrams. 
Two classes of secondary rocks are described: namely, (1) the mechanic- 
ally derived schists, gneisses, and gabbrodiorites, and (2) a series of hydrous 
alteration products, chiefly steatite, chloritite (chlorite-rock), and serpen- 
tine. 
The vast majority of occurrences, while more or less altered, are essen- 
tially fresh primary rocks. This is especially true of the pure olivine- 
rock, dunite, which is the most common type. Steatite and chloritite are 
pretty widely found, but serpentine is practically confined to a region 
within fifteen miles of the French Broad River. Even here remnants 
of unaltered peridotite are abundant. 
The various modes of alteration and decomposition are described in 
chapter iv. Five distinct processes are recognized, and are designated, 
except the first, by the prevailing product; namely, (1) weathering, (2) 
serpentinization, (3) steatitization, (4) chloritization, (5) amphibolization. 
All of these processes occur more or less together over wide areas, but one 
or another usually greatly predominates. Hence various areas are char- 
acterized by ocherous weathering products or by the abundance of one 
of the minerals, serpentine, talc, chlorite, and amphibole, with smaller 
proportions of the others. 
The long-vexed question of the origin of the peridotites is discussed in 
chapter v. A historical sketch shows the kaleidoscopic variety of opinions 
and hypotheses that have been advanced to account for these rocks since 
1875, the date of Professor Kerr’s first report on this region. By various 
authors they have been regarded as unaltered sediments, metamorphic 
sediments, chemical deposits, metasomatized limestones and schists, and 
as igneous intrusions. Opinions have been divided chiefly, however, into. 
two groups, corresponding closely to the old Neptunian and Plutonic 
schools of geology. The strong modern tendency toward the igneous. 
theory of origin is clearly shown, and the correctness of this view is abun- 
dantly substantiated by this report. The data presented on this point are 
grouped under five heads, as follows: (1) mineralogic characters, (2) 
microscopic characters, (3) gross structures, (4) modes of occurrence, (5) 
relations to the gneisses and schists. 
In the discussion of the general petrology of the basic magnesian rocks, 
the genetic unity of the series throughout the eastern crystalline belt is. 
