1874.] 249 [Allen. 



even black. The purple and olive tints are quite frequent, while the 

 general mass is often beautifully banded with narrow zones of several 

 of the above named colors. The texture varies from a glazed, vitreous 

 or porcellanic, compact outer surface, and a dense, jaspery inner struc- 

 ture, with conchoidal fracture, to that so porous and vesicular as to 

 float on water, while every degree of porosity between these two ex- 

 tremes can also be found. The vesicular portions are usually black, 

 but are sometimes grayish, and occasionally every shade of red is pre- 

 sented, from dark reddish brown to bright carmine. These highly 

 variegated beds are usually but a few inches to a foot, or perhaps a 

 foot and a half in thickness, and are found only in certain localities 

 where the clays before induration doubtless contained the peculiar 

 elements that have given rise to these varieties of color. The natural 

 surfaces usually present a glazed, waxy or vitreous, or sometimes a 

 pearly, lustre ; a fresh fracture usually has a jaspery appearance, but 

 quite often also exhibits a waxy lustre. Above these thin, variegated 

 beds occur the reddened, baked clays, which may present a thickness 

 of four to twenty feet, and hence from their thickness and the uni- 

 versality of their occurrence form the characteristic feature of these 

 regions of metamorphism. The color resembles that of bright red 

 bricks, and where the material has been thinly scattered about by the 

 gradual demolition of the buttes it once covered, the resemblance of 

 the locality to an old long-abandoned brick-yard is very striking. 

 These reddened beds are extremely fissile, breaking up into small, 

 irregularly shaped splinters and fragments, and possess a metallic 

 resonance. Nearly all these beds contain impressions of plant re- 

 mains, chiefly stems and blades of broad leaved sedges and grasses, 

 too imperfectly preserved to be of value as specimens. Occasionally, 

 however, one meets with quite well preserved impressions of the 

 leaves of exogenous trees. The sands that next overlie these beds of 

 hardened clay are also generally affected to a greater or less degree, 

 being baked into a red, coarse-grained, generally rather soft sandstone, 

 hand specimens of which can be selected that are unclistinguishable 

 in appearance from the red sandstone of the Connecticut Valley. 

 When the clay beds are very thick the metamorphism sometimes alto- 

 gether fades out before the overlying sandy strata are reached ; in 

 other cases, where they are thin, ten to twenty feet of the superim- 

 posed sands may have been converted into a bright red, coarse, 

 rather friable sandstone. 



The quantity of explosive gases disengaged by the burning of the 



