of' Siliceous Stalactites. 311 



the cavity, will readily be allowed, since the Neptunian cha- 

 racter of calcareous spar cannot be disputed ; but what pre- 

 vents us from drawing the inference that the rock-crystal 

 reached its station under similar circumstances ? 



Silicified wood occurs in alluvial soil, in such circumstances 

 as to warrant the conclusion, that the petrifying matter was 

 brought into its present situation by the agency of water. 



But the proof that siliceous matter is soluble in aqueous 

 menstrua is presented to us in the most unequivocal manner 

 by gramineous vegetables. In the epidermis of these plants, 

 the silica is arranged in a symmetrical manner. It seems ob- 

 viously to be a natural secretion of the plant in the construc- 

 tion of its ordinary integuments. Sometimes it occurs as a 

 morbid secretion in the joints forming the well known taba- 

 sheer. 



The preceding facts leave no room to doubt that silica is 

 found in nature in the state in which it appears to have been 

 deposited from a solution ; and the following observations 

 seem to countenance the conclusion. 



In this neighbourhood, the prevailing rocks belong to the 

 trap family, and consist principally of amygdaloid, clinkstone, 

 greenstone, and compact felspar, as subordinate to the old red 

 sandstone. Where quarries are opened in these rocks, the 

 rents near the surface are numerous, and form small cavities, 

 the walls of which are occasionally covered with calcareous 

 and siliceous stalactites, though always in thin crusts. In 

 some cases, the liquor from which these have been produced 

 seems to have been small in quantity, to have been collected 

 in one spot on the roof, and to have left a thin film of the 

 earthy matter not larger than the nail of the finger. In other 

 cases, the surface covered is of greater extent, but the thick- 

 ness of the matter never reaches a quarter of an inch. The 

 siliceous matter seems to have dropped from the roof of the 

 cavity, in some instances, on the fragments of rock, on the 

 floor, forming stalagmites. 



The surface of this stalactitic crust is rough, and, where 

 thickest, it rises into numerous blunt mammillary processes. 

 In the siliceous portions, these processes acquire a degree of 

 hardness, translucency, and smoothness of surface, approach- 



