292 The American Geologist. November, igno 
suit of the transformation of the atigite. It is very rare in the 
sections studied. 
Honatite is extremely abundant, but is nearly confined to 
the orthoclase. It occurs in very minute particles and flakes, 
and is red by transmitted light as w^ell as reflected light. 
Epidotc occurs in small anhedra. It has a pale green color 
and determinable pleochroism as follows: 
;?g = yellowish green 
«m ^yellow (greenish' 
;/p = pale yelluw 
The absorption is: -'/g >//m>//|) . 
The refringence is very high, and the birefringence strong 
with : 
%— //m =^ .012 at least, 
//m— //p = .025 at least. 
n^ — ;/p =: .037 at least. 
Titanite is very rare. It occurs in irregular grains without 
any good cleavage. The birefringence is very strong; but;/m- 
//p =.008 at most. From the measure of 2d, the angle of the 
optic axes (2E)^40° about. 
Quartz and calcite are rare alteration products, only found 
in highly altered areas. 
The CHEMICAL COMPOSITION of the orthoclase gabbro shows 
that the rock is unusually rich in titanic acid; which is proba- 
bly to be attributed chiefly to the magnetite; the magnesia is 
low, and the manganese quite notable. The phosphoric acid is 
abundant; the potassium undoubtedly belongs to the ortho- 
clase, while the relative amounts of calcium and sodium show 
that the dominant feldspar is labradorite. Streng's analysis 
(II) is added for comparison. 
It is evident from the mineralogical and chemical compo- 
sition that this rock is, in a certain measure, to be compared 
with a monzonyte. It possesses certain characters of the mon- 
zon}"tes, stich as low per cent of magnesia, richness in calciimi, 
of which there is an excess, and relatively high content of al- 
kalies; however the latter are in les^ quantity than in the mon- 
zonytes, and the sodium oxide exceeds the potash, in spite of 
the presence of orthoclase. whereas the reverse is characteris- 
tic of monzonyte. 
