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118 J. D. Dana— Origin of the Rocks of the Cortlandt Series. 
dioryte and then back again to slate, in the most irregular way. 
The idea of an eruptive origin is utterly out of the question ; 
and that of a “ voleanic-ash” origin for the material has nothing 
to sustain it, since not even one small dike of igneous rock or any 
other evidence of igneous eruption older than Triassic has yet 
been found within a circuit of fifty miles; and what there 
are of veins in the older rocks are made of granitic or siliceous 
material. Since these isolated portions of massive labradioryte 
are parts of a stratum lying directly beneath the limestone: 
horizon, which stratum would be likely to be more or less cal- 
careous through an organic source, the lime of the labradorite 
in this rock may be only the calcareous portion of the original 
sediments; and what additional soda was needed may 
come from the permeating brine water. i ay 
illustrate the mode of origin of other metamorphic labradorite 
and oligoclase rocks. 
e hypothesis that the massive Cortlandt rocks were made 
by the above-explained method—that is from “ ordinary detri- 
tus supplemented by materials from the ocean ”-—is therefore 
not wholiy improbable. It is still less so when some details 
connected with it are considered. 
ing sea-marshes prevailed, or alternated with open seas, over the 
