MUSEUM OF COMPARATIVE ZOOLOGY. 7 
about them. Both varieties have suffered alteration to the ordinary 
product, chlorite. 
Professor Wadsworth considered the biotite secondary to the augite, 
chlorite being an intermediate stage in the process of alteration. It 
seems, however, much more probable that this form of the biotite, if 
indeed secondary, is derived directly from the augite, and that the fur- 
ther alteration of biotite to chlorite sufficiently explains the occurrence 
of the latter between biotite and augite. The occurrence of biotite asa 
pseudomorph after augite has been described by Blum,? Richthofen,? 
Tschermak,* Rohrbach,* and Brauns.- On chemical grounds, without 
assuming a high degree of metamorphism, the change from chlorite to 
biotite is difficult to conceive. In some sections, particularly No. 207, a 
large part of the chlorite can be referred to the diabantite of Hawes.® 
In many slides chlorite occurs in clearly defined hexagonal sections sur- 
rounded by one, or more frequently four or five, concentric rims of mag- 
netite. In other cases biotite can be seen in these basal sections in the 
act of changing to chlorite, the centre of the crystal being biotite, about 
which is a wide or narrow rim of chlorite. Figure 1 is taken from sec- 
tions No. 202 and No. 203, and shows the different stages in this process 
of pseudomorphism. 
Apatite is found as a constant constituent, in unusually large clear 
crystals, cutting all other minerals. A very small amount of quartz is 
present, which, in some cases at least, is of secondary origin. Pyrite, mag- 
netite, and ilmenite are present in varying amounts. Magnetite is either 
in hexagonal sections or more or less irregular masses. These masses are 
often elongated parallel to blades of chlorite, and are then evidence of 
secondary origin. A case of this kind is shown in Figure 2. Ilme- 
nite appears in sections, generally hexagonal, like the magnetite, but is 
easily distinguished by its change to leucoxene or titahite. In a section 
from the Granite Street quarries (No. 207) this change has been com- 
plete and the only vestiges of ilmenite are the masses of white, more or 
less opaque, highly refracting leucoxene. In other specimens (Nos. 202, 
208, 209 a) the decomposition has been less complete, but has taken place 
‘in bands, which have three directions parallel to the sides of the rhom- 
1 Pseudomorphosen, I Nachtrag, p. 30; III Nachtrag, p. 93. 
2 Wien Akad., XXVII. 335. Blum, Pseudomorphosen, III. 96. 
8 Porphyrgesteine Oesterreichs, Wien, 1869. 
4 Min. u. petr. Mitth., VII. 27. 
5 Neues Jahrbuch, V Beilage Bd., 275. 
6 Mineralogy and Lithology of New Hampshire, p. 120. 
