428 Dr. Mac Culloch on the Geology of 



on this question. Thus we may possibly find that this disputable 

 appearance forms part of a series of very common phenomena, and 

 that although it may possess a certain general resemblance to the effects 

 of mechanical action, it is, in fact, produced by chemical agencies. 



There are two circumstances to be regarded in describing the frac- 

 ture and texture of a rock, its small or artificial fracture and micro- 

 scopic texture, and its large or natural fracture and texture, as they 

 are determined and exhibited by the effects of time or decomposition. 



The instances which might be adduced to illustrate these differ- 

 ences are sufficiently familiar. In the case particularly of the large 

 disposition and fracture of rocks, a striking difference occurs in the 

 often approximated beds of trap and sandstone. Both form parallel 

 beds, and both on the application of force break into nearly similar 

 fragments, yet the natural division of the sandstone bed shows an 

 horizontal tendency, or one parallel to the plane of the bed, while 

 that of the trap is veitical to it. From the vertical fracture a series 

 of gradations occurs which at length assumes the perfectly geometrical 

 form of polygonal columns. In this case then, we have a form still 

 more perfectly mechanical and regular than that of the most even 

 stratification, and produced by a species of crystallization, a tendency 

 to decided forms in those particular rocks, with the laws and causes 

 of which we are at least as well acquainted as we are with the laws 

 that determine the figure of a quartz crystal. At present they are 

 both equally inexplicable. There is no further difficulty in con- 

 ceiving that a rock may constitute a huge bed separable into hori- 

 zontal laminae as regular as the strata of a mechanical deposit, than 

 in conceiving that the island of Staffa is separable into columnar 

 fragments, or the rock of Devar into vertical laminse. It is true that 

 we have not yet produced any instance of continuous, horizontal 

 laminar concretions, which are incontrovertibly not mechanical, yet 



