REVIEWS 89 
Chapter v closes with a discussion of the secondary rocks. ‘The authors 
undertake to trace back to their original types the various laminated and 
hydrated derivatives. The question arises whether the amphibolites, 
diorites, hornblende-schists, and hornblende-gneisses, may not themselves 
have been derived from corresponding pyroxenic types, such as occur in 
the Maryland and Delaware gabbro areas. The fact that undoubted 
gabbrodiorites do occur in portions of the belt in North Carolina makes 
it quite probable that many, if not all, of these amphiboliferous types 
have had a like origin. 
Chapter vi deals with the mineralogy and technology of corundum, 
including its crystallography, its physical and chemical properties, its 
applications in the arts, and an outline of the process of manufacture of 
the several types of corundum and emery wheels on the market. 
Chapter vii, on modes of occurrence, shows corundum to be a constit- 
uent of a remarkable number and variety of rocks, including nineteen 
igneous types, nine metamorphic, and one unaltered sedimentary. These 
corundum-bearing rocks are distributed as follows: 
CORUNDUM-BEARING IGNEOUS ROCKS 
North Carolina Other American Localities Elsewhere 
Peridotite Granite Kyschtymite 
Pyroxenite Syenite Diorite 
Amphibolite Nephelite-syenite Tonalite 
Anorthosite Plumasite Gabbro 
Pegmatite Norite Trachyte 
Andesite Quartz-porphyry 
Monchiquite Basalt 
CORUNDUM-BEARING METAMORPHIC ROCKS 
Serpentine Crystalline limestone Corundum-schist 
Gneiss Corundum-porphyroid 
Mica-schist Graphite 
Quartz-schist Igneous contacts 
Amphibole-schist Inclusions 
Chlorite-schist 
OTHER CORUNDUM-BEARING ROCKS 
Alluvial gravels 
Undetermined (emery) 
The American occurrences, particularly those of North Carolina, are 
described in detail and compared with similar deposits, when known, in 
other parts of the world. Those of chief commercial importance in North 
Carolina are in peridotites, and to a less extent in amphibolites and 
pyroxenites. The gravel deposits are of interest on account of the corun- 
dum gems (rubies) and the garnet gems (rhodolite) that occur in some of 
them. 
In peridotites corundum occurs chiefly (1) in peripheral or border 
