J. D. Dana—Helderberg rocks in the Connecticut Valley. 339 
The “settling” effect of alum, however, appears to be mainly 
due to the precipitation of alumina by the carbonates of lime 
and magnesia, present in almost all sediments. 
The remarkable action of lime, in preventing diffusion and 
diminishing the plasticity of clay, will form the subject of a 
future communication. 
No. 173. Under-subsoil of Cretaceous prairie, Monroe Co., Miss. (See Miss. 
. 262). 
»?p 
Time of boiling. h 30 h. 
> 64™™ h. v. (bog ore) 2°10 2°07 
S56" (piliceous wand)... ccc cc on 0°62 0°55 
ge ou 0°20 0°21 
i i a 1°26 1-21 
Qh & 5-18 2°92 
1“ 46 6°30 7-36 
: 13°19 8°81 
“ 27°93 7°85 
<0°25 27°02 35°22 
Clay, 14°82 33°16 
98°42 99°36 
hove eee 
Art. XXXVII.—On Rocks of the Helderberg era, in the valley 
of the Connecticut—the kinds including Staurolitic slate, Horn- 
blendic rocks, Gneiss, Mica schist, etc., besides Fossiliferous Lime- 
stone; by JAMES D. DANA. 
IN my 
Stratigraphical grounds, that the metamorphic rocks, gneiss, 
mea schist, mica slate, chloritic mica slate, chlorite slate and 
others occur, underlying and overlying quartzite, 1n 07 the 
Lower Silurian ormations; and also that staurolite crys- 
tals abound in a mica schist of the same age. 
Th the following pages, it is further demonstrated that not 
kd the same kinds of metamorphic rocks, but even horn- 
lende rock and schist and syenitic gneiss are extensively devel- 
In a formation of Helderberg age, and probably the Upper 
“elderberg or Lower Devonian. Moreove 
lite are here also abundant; so that this mineral species, which 
i ood as a_ fossil 
f me American 
che wstinguishin age, 
Chometric virtues only of a species of immensely wide range. 
; ‘ 
