p 
| 
ones except serpentine, and its congener rensselaeri 
Emmons on American Geology. 399 © 
80 as to take the shape and position of pyroplastic rocks ; indee 
it would be difficult to prove that any of these latter rocks have 
any other origin than the fusion of subjacent hydroplastic strata. 
_ The pyrocrystalline limestones are introduced to our notice by 
- Emmons in the following language, /iteratim: 
“The class of limestones under consideration, though they contain 
many minerals, yet as a rock it is not associated with any important 
i i - ge ae ye 
cumstances under which this rock occurs in this country, warrants its 
Tecognition as a rock quite as distinct from all others as granite. It is 
y no means a metamorphic mass.”’—p. 57. 
Having mentioned the limestones of Northern New York, and 
those of the southern counties of the same State, he proceeds to 
object to the view that they are Silurian strata altered by heat. 
In order to sustain his own notion of the igneous origin of erys- 
talline limestones, Mr. Emmons confounds, perhaps ignorantly, 
the limestones of two different regions occurring under very un- 
like conditions. Those of Southern New York and the adjacent 
parts of New Jersey and Pennsylvania have been clearly shown 
by Rogers and by Mather to be of Silurian age, while the erys- 
talline limestones of Northern New York belong to a formation 
older than the Potsdam sandstone. Mr. Hunt, of the Geological 
urvey of Canada, in a paper on the crystalline limestones of 
Canada and the Northern States, read before the American Asso- 
Ciation at Washington, in May last, and published in this Journal 
for September, has clearly pointed out the facts in these two 
cases, and has shown that the different limestones cannot for a 
moment be confounded; and that they have nothing in common, 
but the crystalline minerals which belong to the altered lime- 
Stones of all geological ages. In Northern New York, as Mr. 
Hunt has sho 
feldspar rock, having generally the composition of andesine or 
group to which the crystalline limestones belong, while there is 
nothing to re t these among the altered Trenton limestones 
of Southern New York. - 
