386 J.D. Dana=Ni ote on the Bernardston Helderberg Formation. 
quartzyte was made; and hence that the region, when the 
deposition took place, was not the border of the open se 
7. Origin of the Rocks.—To understand the rocks of this 
Helderberg region, it must be borne in mind: that quartzyte 
beds in their original state, that is, beds of quartz sand, may 
have been formed at different times in the course of the era, 
owing to changes of est or of currents; that the sand beds— 
like those of any other era and of the present time—would, in 
many places, have had more or Jess eartby material (ground-up 
crystalline rock) with the quartz sand, so that metamorphism 
ould not make pure quartzyte out of it, but might make a 
micaceous or gneissoid quartzyte, ora quartzytic gneiss, accord- 
ing to the nature of the earthy material present; that while 
sand-beds were formed where the currents were rapid enough 
for the purpose, mud-beds would have formed where the waters 
mica rock, and the hornblende rocks wou iave een pro- 
duced. The existence of some potash and alumina in the 
triturated rock or mud (both ingredients of orthoclase) would 
have favored the formation of brown mica (biotite) by meta- 
the same proportion in the blende as in the mica. Anal- 
yses of average biotite and dark green hornblende afford : 
Biotite. Hornbiende. 
J RR Seas ae ce a 45 
PRU 2a ce es a 18 * 10-12 
Iron protoxide 
eee proto t ice sis es ap 
Magneto 2 See Ss: - 22 20 
We 20 Set ee AL ae 14 
WORMS Fe i ct 10 
The magnesia would have come from the trituration of such 
older rocks as are made partly or wholly of minerals contain- 
ng nt hg which tise. hornblende and biotite are the most 
co 
‘Admittitig the Cods formation of Professor baptipe 3: and 
the Calciferous mica schist adjoining it, to be of the same 
formation with the mica slate, headed bat and borhktinds rocks 
of the Bernardston and Vernon region, which Professor H. 
states to be a fact, this Helderberg Somnitien stretches north- 
ward beyond the ‘boundary of New England, with a breadth 
along the Connecticut Valley of fifteen to thirty miles or more’: 
