198 Wisconsin Academy of Sciences , Arts and Letters . 
as wholly to replace the crystal, in other cases the chlorite occupies but 
one half of the crystal or appears in inclosed areas within the crystal. 
The chlorite is strongly pleochroic (yellow to green), and is often radial 
(delessite.) Both hornblende and augite are frequently twinned. The 
small amount of quartz not in the form of micro-pegmatite may be either 
original or secondary. The black ore material shows in some cases in 
square sections and is strongly attracted to the magnet in the rock powder. 
Yet many grains are almost entirely changed to leucoxene, indicating 
the presence of titanium, probably as ilmenite. Apatite is present though 
not in great abundance. 
Section 2, from a specimen taken at a point about ten feet nearer the 
contact, shows besides a more finely crystalline texture than section 1, 
certain differences in mineralogical composition. It contains more 
quartz than micro-pegmatite. The feldspar is more altered. The mag¬ 
netite shows a greater tendency to appear in skeleton growths, and well 
crystallized sphene, as well as considerable leucoxene is present. As in 
specimen 1 the augite is but little altered, while the hornblende is in 
many cases much decomposed to chlorite. 
Section 3 shows very beautiful skeleton forms of magnetite, long and 
slender growths running entirely across the field with a magnification of 
thirty. Imperfect parallel growths of brown hornblende about augite 
are seen much as they have been described in the augite diorite from 
Somerville, Mass.,* with which this rock presents many analogies. Like 
section 2, this section contains sphene, and epidote was made out, prob¬ 
ably a product of the combined alteration of feldspar and hornblende. 
The characters of this rock as made out in the three sections agree well 
with those of the camptonites of Rosenbusch. 
In section 4, from the contact of the igneous intrusion with meta¬ 
morphosed green shale (locality 3 on map), the decomposition is too great 
for the section to be of great interest. It can be seen, however, that the 
feldspar was in lathshaped crystals with perfect outlines, oriented with¬ 
out reference to one another. The structure is that of a typical diabase. 
Between the altered feldspar laths is a mass of decomposition products 
of a greenish or dirty brown color in which chlorite is made out, and 
the mass is crossed by serrated magnetite skeletons. No distinct crys¬ 
tals of either hornblende or augite can be made out, and it seems prob¬ 
able that both hornblende and augite are entirely altered. 
In order to form reliable conclusions as to the dependence of the rock 
characters on the distance from the contact, observations should be made 
on a larger number of sections at several localities somewhat widely sep¬ 
arated. The evidence obtained so far as it goes, seems to be, (1) that 
* there is a gradation in texture, the finest grained rock being nearest the 
* Wm, H. Hobbs, on the Petrographical characters of a Dike of Diabase in the Boston 
Basin: Bull. Mus. Comp. Zobl., Harv. Coll. xvii. .Plate 1, fig. 2, March, 1888. 
