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 as a 

 pseudomorph after augite has been described by Blum,-^ Eichthofen,^ 

 Tschermak,^ Eohrbach,* 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 Xo. 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 sorae 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 titanite. 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- 



^ Pseudomorpliosen, I Nachtrag, p. 30; III Nachtrag, p. 93. 

 2 Wien Akad., XXVII. 335. Blum, Pseudomorpliosen, III. 96. 



* Porphyrgesteine Oesterreichs, Wien, 1869. 



* Min. u. petr. Mitth., VII. 27. 



* Neues Jahrbudi, V Beilage Bd., 275. 



6 Mineralogy and Lithology of New Hampshire, p. 120. 



