MINERALOGY. 75 
on Pl. 4 is drawn from one of these garnets, and shows it as it appears 
when magnified twenty diameters. When this section is examined be- 
tween crossed Nicols, the garnet becomes black, but the quartz springs 
out into brilliant colors. The garnet does not enclose any of the horn- 
blende of the rock, but it does sometimes enclose a piece of magnetite. 
This seems to be a remarkable case of impurity, which reminds one of 
the Fontainebleau limestone, which is carbonate of lime, the crystals of 
which sometimes contain half their weight of sand. In our garnets, 
however, the quartz, being transparent, does not become evident until a 
thin section is made; and no Better illustration than this can be found, to 
show the value and necessity of microscopic work in connection with 
mineral determination, since experience shows that analyses made on 
apparently pure material may be worthless, on account of the presence 
of weighty impurities. 
Garnets are common in the clay slates of the Connecticut valley. In 
them a very pretty process of pseudomorphism can be seen in progress, 
which is represented in Fig. 5 on Pl. 7. Here a garnet perfect in out- 
line is slowly changing into chlorite. The chlorite in this specimen is 
arranged concentrically about the garnet in foliated masses. In other 
specimens that I have seen, from other localities, the foliz of chlorite 
were arranged radially. Prof. R. Pumpelly* has described and figured 
garnets from the Lake Superior region that were almost entirely changed 
into chlorite. The garnets in these slates also contain some quartz, but 
not as much as the Hanover crystals. 
48. Zrrcon [Zr Si Ox]. 
This mineral is found as a microscopic constituent of some of our 
granites and sienites, but I am not aware of its occurrence in macro- 
scopic crystals. Zircons in the granite are not very common, but the 
crystals, though very minute, are often perfect in form. Fig. 1 on Pl. 5 
represents some crystals of zircon in the Fitzwilliam granite. They are 
highly magnified. As is seen, some of the crystals show the perfect 
quadratic base, and others show the prism. Again: some of the crystals 
are rounded, and yet approximate to the form of zircon. The dark min- 
eral on the sides ‘of the figure is biotite. The zircons are bedded in 
* American Fournal of Science, iii, vol. x, p. 17. 
