850 J. D. Dana—llelderberg rocks in the Connecticut Valley. 
If this conformability is sustained by further observations, we 
shall have here the additional lithological fact that— 
Chlorite rocks, including protogine, may constitute meta- 
morphic beds of Helderberg age. 
8. The facts which have been presented sustain the statement, 
made on an early page of this article, that lithological evidence 
of geological age is to be distrusted. If, when used by one who 
has made it a special study, it leads to the conclusion that true 
Helderberg rocks in the White Mountain series are of Cambrian 
or earlier age, it is surely a bad reliance. If it also makes out 
that Green Mountain rocks are of Huronian age, or of some 
other pre-Silurian period, when in reality belonging to the later 
part of the Lower Silurian, geologists may well be afraid even 
of its suggestions, unless it have sure stratigraphical support. 
“Huronian” areas have been defined in various parts 0 
country and the world, on the basis of this evidence alone ; and 
in such cases who knows any thing whatever with regard to the 
real age of their rocks? It is probable that the rocks consid- 
ered characteristic of the Huronian occur also among true Lan- 
rentian terranes; it is certain that they do in formations later 
than the Lower Silurian. 
Finally, what reason is there in chemistry or geology why 
crystals of andalusite and staurolite should have been made only 
in pre-Silurian time? Andalusite consists simply of silica and 
alumina, and staurolite of the same, along with iron. ese 
three ingredients are now and ever have been the most abun- 
dant of all the mineral constituents of the globe. With or with- 
out the iron, they are the materials of all clay deposits; an 
clay deposits from the decomposition of granite, gneiss and 
related rocks have been forming over the globe, and increasing 
in amount, ever since these rocks began their existence. 
They are therefore the very last minerals that should be 
thought of as pre-Silurian “fossils.” Were zirconium oF in 
other of the rare metals a constituent in ies, there 
all time. These remarks apply equally to chlorite, 1D er 
these three ingredients occur, with only a little magnesia beside, 
and water, 
9. The epoch of disturbance, in which these Helderberg 
lized, 
allization of Devonian and older rocks took place ov 
runswick and Nova Scotia. It was the most prominent ak 
of mountain-making on this eastern border of the contm 
