ALTERED SANDSTONES. 91 



grains. Both of these minerals are authigenetic. There are also decompo- 

 sition pi'oducts, including a little serpentine. 



No. 134, Sulphur Bank, is manifestly a much altered, arenaceous rock, 

 of a strong green tinge, intersected by minute veins of a feldspar-like min- 

 eral. Under the microscope the clastic character is perfectly distinct, the 

 clear or somewhat milky grains being divided by a net-work of thoroughly 

 characteristic conformation. The net is composed of green and brownish- 

 green matter. The original clastic minerals ha^•e entirely disappeared. 

 The thin section shows half a dozen minute veins, more or less continuous, 

 and these are filled with feldspar, which is for the most part granular, 

 but occasionally shows irregular, hemitropic lamellae. On the course of one 

 of the veins, at a point at which the vein pinches, is a very remarkable 

 feldspar aggregate of lath-like microlites extending over an area several 

 times as wide as the adjacent vein. This aggregate has replaced, wholl)' 

 or in part, several clastic grains the shape of which is faintly ti'ace- 

 able by brownish streaks. Three-fourths of the peripher}- of the aggregate 

 is nevertheless bounded by straight lines. The aggregate is composed of 

 polysynthetic, latli-shaped microlites almost exactly parallel to one another 

 and giving low angles of extinction on each side of the twinning plane. 

 The position of these microlites corresponds to the straight outlines and 

 the entire aggregate appears to represent a porphyritic, authigenetic feld- 

 spar cut nearly perpendicular to the brachypinacoid and showing two 

 distinct terminal outlines at one end. The cliief distinction between this 

 and porph3'ritic feldspar in eruptive rocks is the frequent interruption of 

 the hemitropic lamella?. The traces of the original outlines of the clastic 

 fragments still visible in tlie feldspar forbid the supposition that it is an 

 allothigenetie mineral; neither is it possible that such a crystal should have 

 been transported and redeposited without abrasion of its corners. The 

 slide shows several other feldspars of similar character, but less perfectly 

 developed. The greenish-brown net-work between the altei-ed grains in 

 this slide is chiefly composed of a non-dichroitic, fibrous mineral show- 

 ing dull-yellow interference colors and tlms corresponding to serpentine. 

 There are no patches of it large enough to show the characteristic grate 

 structure. 



