.^04 The American Geologist. November, 1900 
darker, often greenish in its fresh condition, while the weath- 
ered surface has a conspicuous yellow-brown, rusty color. 
These exterior distinctions are accompanied by still more 
im,portant differences in chemical and mineralogical composi- 
tion. The most important and abundant mineral in the g..b- 
bro is the plagioclase; in the noryte it sometimes becomes so 
uncommon as to be scarcely more than an accessory con- 
stituent. At the same time it frequently becomes more acid 
in character. The olivine, often abundant in the gabbro, is 
entirely lacking in the noryte, while the augite of the gabbro 
is replaced by the pure ferro-magnesian orthorhombic 
pyroxenes. This mineralogical transformation indicates 
greater relative richness in iron and msgnesia, and compara- 
tively little calcium oxide in the magma ; conclusions fully 
borne out by the chemical analysis. 
But various new minerals also appear. Biotite, so far as 
known, only occurs in the gabbro as a result of alteration 
usually of the mesogenetic type. In the noryte on the other 
hand it is one of the common primary minerals, indicating the 
presence of some potassium not in the feldspar. And, in fact, 
the potash is perceptibly higher in the noryte than in the gab- 
bro, while the excess of alumina is very marked. There can 
be no doubt that the biotite is of primary origin since it is not 
infrequently surrounded by cjuartz, clearly also derived directly 
from the magma. The latter mineral furnishes another dis- 
tinguishing feature since it is almost unknown as a primary 
mineral (and rare as a secondary product) in the gabbro. But 
the most interesting among the new minerals is the cordierite, 
which forms a notable per cent of the rock. Its presence is 
not only significant and interesting, but it is entirely excep- 
tional both from a mineralogical and petrographical point of 
view. In fact, until within a few months, this mineral, so 
abundant in certain granites and metamorphic rocks, was un- 
known in basic granitoid rocks, and only two or three* oc- 
*These occurrences are: in the "Kersantite" from Michaelstein in 
the Hartz. described by Max Koch (Jahrb. k. preuss. geol. Lande- 
sanst. Berlin. 1887. p. 44.); in the "cordierite-vitrophyrite' from near 
Harri.smith (Orange Free State), described by M. Molengraaf (N. 
Jahrb. f. Miner, etc. I. 1894. p. 7q) : and MM. Lacroix (Les Enclaves 
des Roches volcaniques), Prohaska (Sitzb. k. k. Akad Wissensch. 
Wien, 1883, XVIIL), and Zirkel (Neues Jahrb. f. Min. etc., 1891, I. 
p. 109.) have described coHierite produced in sandstones enclosed 
in basalitic rocks, or in contact with the latter. Ref. Lacroix: Le 
Gabbro du Pallet: Bull. Carte Geol. France. No. 67. (1899) p. 46. 
