PYROCRYSTALLINE LIMESTONE. 
83 
than those of New Jersey and New York, which contain some 
of the rarer minerals, as zircon, sapphire, spinelle, and brucite. 
Still all these limestone beds must be regarded as belonging to 
the eruptive class. Those which are found in the first three 
states mentioned, resemble the beds in the Hoosick range, 
which pass through the eastern part of Berkshire and western 
part of Hampshire in Massachusetts. We should at any rate 
not confound them with the Vermont and Berkshire marbles, 
which belong clearly to the sedimentary series, and which are 
continuous and persistent through areas of great length. It is 
perhaps not easy to distinguish them in hand specimens, or in 
the cabinet, but their associations in the field attest the forma¬ 
tion to which they belong. Geographical position often 
obscures the relations. For example, the beds of dolomite in 
Dalton and Washington are pyrocrystalline or primary lime¬ 
stones, while those of Pittsfield, only four miles to the west, 
belong to the Taconic system, or to the sedimentary class. 
The rocks can not be distinguished from each other by their 
lithological characters, and both are not unfrequently regarded 
as metamorphic; but the former stand in the same relation to 
metamorphism as granite and gneiss. The latter have no doubt 
undergone a change in their lithological characters, but it is 
not necessary to infer that the agent which induced the change 
was heat. Those geologists who refer all changes in structure 
and texture to heat, take only a partial view of the forces 
which act, and which have acted upon the earth’s crust. A 
comprehensive view of the cause of structural change in rocks 
is of great value in geological reasoning, and we are thereby 
enabled to account for those changes when collateral facts forbid 
the agency of fire. The view which I have presented of the ori¬ 
gin of those masses of limestone so common in St. Lawrence, 
Essex, and Orange counties, New York, in modified forms, two 
of which are prolonged into southern states, is consistent with 
known facts. 1. The position of many of these masses is such 
that they can not be referred to the lower Silurian limestones, 
as has been attempted by several eminent geologists. 2, From 
