158 
Wisconsin Academy of Sciences , Arts and Letters. 
blende. The alteration begins on the surface of the pyroxene and proceeds 
from there toward the center. From the same hand specimen sections 
were prepared in which could be seen almost unaltered pyroxene, and the 
same mineral with wide marginal fringes of hornblende. In the hand 
specimen the hypersthene-gabbro possesses a bronzy lustre from the pre¬ 
ponderance of orthorhombic pyroxene, while the gabbro-diorite is green, 
owing to the secondary hornblende derived from pyroxene. 
An area five miles square immediately southwest of that just described, 
I have examined in the field and studied by means of microscopic sections 
and otherwise in the laboratory. The western portion of the area is occu¬ 
pied by the Pre-Cambrian gneisses. The remaining portion of the area 
contains outcrops of gabbro, gabbro-diorite, true diorites, hornblende 
gneiss, and intrusive rocks which belong to later periods of eruption. The 
field study shows that the gabbro forms a continuation of the gabbro area 
studied by Professor Williams. This part of the gabbro area, however, 
differs in two respects from that farther north; first, in the greater differ¬ 
entiation of the magma, which here produced true diorites as well as 
gabbro; and, second, in the greater metamorphic action, which has pro¬ 
duced from the gabbro first a gabbro-diorite, and then a hornblende gneiss. 
The true diorites have gone over to hornblende gneiss by much the same 
process as the gabbro-diorite. These several modifications are assumed to 
have formed one and the same mag¬ 
ma from the absence of any visible 
contact between them. The gabbro 
and diorites have not been found in 
the same outcrop, but the field evi¬ 
dence is such as to make it very prob¬ 
able that they are differentiations of 
the same mass. Gabbro, gabbro dio- 
rite, and hornblende gneiss on the 
other hand, can be seen to pass imper¬ 
ceptibly into one another in the vicin¬ 
ity of Ilchester. The microscopic ev¬ 
idence is most confirmatory on this 
point, slides having been prepared 
from a sufficient number of speci- between feldspar and hornblende. X 50. 
mens to show the steps in the pro¬ 
cess. To make clear, it will be necessary to give somewhat in detail 
the characters of the end types, gabbro and hornblende-gneiss, and of 
the intermediate varieties. 
The entirely unaffected hypersthene gabbro has not been found within 
the area studied, but a rock containing both hypersthene and diallage sur¬ 
rounded by marginal fringes of green hornblende, composes a core a few 
feet in diameter enclosed in gabbro-diorite. This core occurs in the wall 
of rock formed by the railroad cutting at Ilchester station, and its brown 
