MINERALOGY. 75 



on PI. 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 blaclc, but the quartz springs 

 out into brilHant 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 PI. 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 foliae 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. Zircon [Zr Si O4]. 



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. i on PI. 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 jfournal 0/ Science, m, vol. x, p. 17. 



