ALTERED SANDSTONES. 87 



Garnet. — In tliG glaucopliaiie schists garnets are not infreqvient, though 

 nowhere very abundant, in crystals measuring from 0.3""" to 2""". Tlie 

 garnet is of a pale reddish-brown tint, perfectly isotropic, and iucliidus zo- 

 isite, titanite, and glaucophane. It decomposes to a coarsely foliated chlo- 

 rite. Nothing answering to the kelyphite of Mr. A. SchrauP has been 

 observed, and it can only be supposed that the decomposition has been 

 effected by the action of magnesian waters. Serpentine is also associated 

 with the decomposing garnets, but not under conditions sufficiently pro- 

 nounced in the material at hand to justify any positive assertions as to the 

 relations of the two minerals. 



other minerals. — Zlrcou is commou as au inclusion in clastic grains and is 

 also probably present in some of the glaucophane schists. A careful watch 

 has been kept for other minerals, such as andalusite, dipyre, prehnite, allan- 

 ite, and zeolites. The last occur macroscopically in the New Almaden 

 mine, but have not been observed in the slides. 



ALTERED SANDSTONES. 



In various specimens of altered sandstone one or other process of 

 transformation may be lacking or insensible, but in a number of cases 

 a single slide furnishes an almost complete epitome of the entire series. 

 I do not think it possible to convey to the reader a better idea of the 

 metamorphism of the sandstones than by describing a few typical in- 

 stances. 



Examples. — A good exauiplo ('f an altered sandstone is afforded by a 

 rock from near Knoxville.^ Macroscopically it is a dark-green, fine-grained 

 sandstone, evidently somewhat altered. Seen under the microscope with a 

 low power it appears to be a typical sandstone, with only insignificant 

 changes. With a No. 4 Hartnack it is seen to contain numerous unaltered 



bleudo. The statements applicable to tlie viriilite of the hornblemle from Fraucke's point of view 

 aie true also of the chloritic material derived from augite and which, as we have seen, so often fills 

 feldspathic sections, for these two substances, so imperfectly determined from a chemical point of view, 

 present fundamental analogies in composition. We have been led to regard the epidote inclosed in feld- 

 spar sections not as ijseudomorpbic after feldspar, but as a result of the transformation of chloritic 

 matter. There are, furthermore, numerous instances of this transformation." 



'Zeitschr. fiir Krys. und Mineral., Groth, vol. 6, p. 321. 



''No. 8, Coast Range collection, bed of Jericho Creek. 



