340 J. D. Dana—Helderberg rocks in the Connecticut Valley. 
That I may not be supposed to misrepresent the views of 
‘‘some American geologists,” I here cite a paragraph from the 
observations of Professor James Hall on metamorphic rocks, to 
be found in the third volume of his N. Y. Paleontology (1859), 
p. 98. 
“The imperfect study of these metamorphic rocks which I was 
able to make, while engaged in other duties, during the years 
Advancement of Science, and I am convinced that something of 
this kind will yet grow out of the farther investigation of the meta- 
morphic rocks and their contained minerals.” 
To which is added in a note— 
“Every observing student of one or two years’ experience in the 
collection of minerals in the New England States, well knows 
that he may trace a mica schist of peculiar but varying character 
from Connecticut through Central Massachusetts, and thence into 
Vermont and New Hampshire, by the presence of apres — 
. = e 
ing certainty the geological relations of this rock, as the Jeane 
of Pentamerus oblongus, P. galeatus, Spirifer Niagarenss si 
macropleura, and their respectively associated fossils, do the rela- 
tions of the several rocks in which they occur.” 
If Professor Hall intended, in the last paragraph, to imply pod 
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