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strongly emphasized. It is noteworthy that a closely similar association 
of rock types is found in almost every peridotite locality, although some 
one usually preponderates in every case. Thus peridotites, particularly 
dunite, prevail in North Carolina and Quebec, pyroxenites in Pennsyl- 
vania, while gabbros are abundant in Delaware and parts of Maryland. 
The types represented in the various regions, however, are almost identical, 
and the petrology is closely similar, except in the relative abundance of 
the various types and in mode and degree of alteration. 
Two generations of corundum are recognized. The greater part, 
including all deposits of commercial value, belongs to the first generation 
and represents the excess of alumina in the original magma. Another 
part, occurring in microscopic grains, is an excess of alumina arising from 
the corrosion of anorthite crystals by the still molten magma. This pro- 
cess has produced sheaths of minerals which form the corrosion mantles, 
so greatly developed in some localities, and in other cases entirely replacing 
the anorthite, or the corroding magma, as the case may be, by nestlike 
aggregates of intermediate silicates. 
In discussing the age of the peridotites (pp. 152-59), it is recalled that 
until recently it has been the custom of geologists to refer the whole of the 
Appalachian crystalline belt to the Archaean, or at least to pre-Cambrian. 
Recent work in several regions makes it impossible longer to accept these 
old correlations without other than merely lithologic evidence. ‘Tables 
are given showing possible correlations of the crystallines in areas recently 
investigated, from North Carolina to Massachusetts and the Green Moun- 
tains, and summaries are given of the various conclusions as to age arrived 
at by geologists in different parts of the field. The conclusions of the 
authors of this report may be briefly stated as follows: The intrusion of 
the peridotites was probably contemporaneous, or practically so, for the 
whole region under consideration, from Alabama to Newfoundland. These 
rocks now form a belt of remarkable unity through a region of great oro- 
genic disturbance and intense metamorphism. ‘These facts, together with 
the geologic relations that have been deciphered in some northern portions 
of the belt, suggest the hypothesis that the chief period of intrusion may 
be correlated with the folding movements of closing Ordovician. The 
peridotite belt doubtless marks the axis of most intense disturbance. 
The later orogenic movements, at the close of the Carboniferous, produced 
the widespread lamination of these rocks, and probably gave occasion for 
additional minor intrusions. Much painstaking work yet remains to be 
done, however, in many parts of the field, before any hypothesis concerning 
the age of the peridotites can be satisfactorily established. 
