S. L. Penfield—Chemical Composition of Amblygonite. 295 
of the Upper Silurian series. At some later time, probably at 
the period of the Appalachian revolution, the whole series, 1. e. 
the Hudson River an wer Helderberg formations, were 
uplifted, folded, fractured, faulted and tilted. 
he complicated stratification of Section 3, may perhaps be 
accounted for as follows. First, unconformable deposition of 
Lower Helderberg upon Hudson River beds; then the formation 
of an anticlinal, and at the same time, closely adjoining it, a 
treble fold. The folds by the continued pressure were broken 
off from the anticlinal and thrown into an erect position. The 
same force has also compressed the lowest fold between the 
edges of the anticlinal on one side, and the surface of other 
Lower Helderberg strata on the other. The uppermost layers 
of this fold were by this action ruptured. This is not shown in 
the cut. As the upper portion of the base of the hill has been 
quarried away, the segments of three folds are now in view, 
one above another. The thickness of the contorted strata has 
been much reduced by pressure. 
Art. XXXIX.—On the Chemical Composition of Amblygonite ; 
by SamuEeL L. PENFIELD. 
THE new mineral species triploidite described by Messrs. 
Brush and Dana* is shown by them to be isomorphous with 
wagnerite and closely related in composition to triplite. These 
three minerals have respectively the formulas (Mn,Fe), P,O,+ 
(Mn,Fe) (OH),, Mg, P,O,+MgF, and (Fe,Mn), P,O,+(Fe,Mn) F,. 
rom a comparison of these formulas it is argued (l. c., p. 45) 
that the relation between the minerals requires the assumption 
that the hydroxy] in triploidite must play the same part as the 
fluorine in the other two. : i 
In this paper I wish to show that in amblygonite the 
hydroxyl group is also isomorphous with fluorine, and that in 
chemical composition the original amblygonite does not differ 
m the American and Montebras varieties which have been 
called hebronite. I shall also show that the results of m 
analyses require the adoption of a new formula for the mineral, 
more simple than that previously accepted. For analysis I 
have selected specimens from the three localities in Maine, 
has bee 
lately discovered by Messrs. Brush and Dana, also two varieties 
from Montebras and one from Penig, Saxony, from a specimen 
in the Yale College collection. — 
e analyses are arranged so as to form a series, beginning 
with the one which contains the smallest amount of water. 
* This Journal, III, xvi, 42, July, 1878. 
