274 J. D. Dana on the Quarizite, Limestone, etc., 
CONCLUSIONS. 
I close with the following conclusions as a sequel to those 
mentioned on former pages of thi ol 
XIIL. Lithological evidence as a test of Geological age.—lt is 
above shown that the mica schist overlying the Canaan lime- 
stone is in Sheffield and Canaan stawrolitic. The mineral stauro- 
lite ewists, therefore, in rocks younger than the Stockbridge limestone. 
But the facts which have been presented, besides opposing 
the idea that staurolite is a satisfactory mark of age, also show 
that lithological evidence should be used with extreme caution 
even within a district of a few square miles. For in a distance 
of three miles or less, a stratum of quartzite has been found to 
become one of mica schist, or of gneiss, or of mica slate, garnet- 
iferous or not, or of hydromica slate, or of chloritic mica slate, 
or chlorite slate containing magnetite or not; and in another 
locality, in the course of two miles, the quartzite stratum 
changes to a coarse staurolitic mica schist, full of garnets ; and 
in another, mica schist is replaced by a granitoid gneiss. Had 
I known these facts when I commenced my investigations in 
Green Mountain geology, I should have had much less diffi- 
culty in reaching right conclusions. It is hence evident that, 
in the present state of the science, we should beware of all con- 
clusions as to the age of metamorphic rocks that rest on litholo- 
gical evidence alone, whether they relate to pree-Silurian or 
subsequent time. en every square mile of New England 
has been investigated stratigraphically with thoroughness, we 
shall begin to know something of its geology. 
It should be understood that tracing a formation along from 
mile to mile by means of the rocks, after an understanding of 
their characters and transitions, is a very different thing from 
deducing the age of rocks on the principle that a particular 
kind belongs to a particular age in geological history. 
It follows from the facts reviewed, as stated in an early 
part of this memoir, that there is not strictly any quartzite forma- 
tion in the Green Mountains ; the formation being made up % 
various rocks, and quartzite being not always the predominant one. 
XIV. Age of the Stockbridge Limestone—This mass of lime- 
stone, as | know from personal study, extends continuously 
from Pittsfield to Canaan.* Beyond Canaan it stretches on, 48 
shown by Percival, to a point south of Pawling in Dutchess 
0., New York—making for the total from Pittsfield southward 
i h 
* The one interval not examined when 52 of vol y was printed, I 
have recently been over. eae ee . 
