SKY. GEOLOGY. HYPERSTHENE ROCK. 389 



that ground below which must first have met it in the 

 descent. Possibly the presence of snow at the time of 

 the fall may assist in explaining this remarkable ap- 

 pearance. 



Although scarcely subject to decomposition, the hyper- 

 sthene rock shows an occasional tendency to separate in 

 crusts capable of being easily detached, and no wise altered 

 in hardness or integrity. In this respect it resembles some 

 granites, as well as some of the more common members of 

 the trap family. 



This rock presents a few varieties of composition, 

 the simplest being predominant and the more com- 

 pound rare. The most compounded variety consists of 

 hypersthene, glassy felspar, common felspar, and horn- 

 blende, and the next differs in the omission of the horn- 

 blende. I ought perhaps to add to these minerals oxidulous 

 iron, since it is at times so equally diffused among the 

 other ingredients as to form an integrant part of the rock. 

 This substance indeed often occurs even in the simplest 

 variety, being almost always crystallized in accumulated 

 tetraedrons ; while it acquires by exposure a black polished 

 face, being, like the hypersthene, unchangeable, and adding 

 to that roughness which characterizes the weathered sur- 

 faces. The simplest varieties consist of a mixture of 

 hypersthene with compact greenish felspar, or with felspar 

 of a crystalline and somewhat glassy aspect. These, from 

 the varying magnitude of the constituent parts, are subject 

 to considerable variety of aspect, the crystals or concre- 

 tions of hypersthene being in some cases half an inch in 

 length, in others not exceeding in size a pin's point. In 

 this latter case the rock cannot easily be distinguished 

 from a common greenstone, where the fracture is fresh ; 

 but the distinction is in general made with great facility 

 on the weathered surface, in consequence of the persist- 

 ence of the hypersthene. In a few instances the felspar 

 is of a dark purplish hue like that of Labrador, and the 

 appearance of the fresh rock is then still more deceiving, 

 although the same natural analysis almost always discovers 



