Prismatic Structure in Sandstone, fyc. 261 



indurated where it approximates to the columnar parts. These 

 are, in many places, extremely hard ; but that character is not 

 universal, and the causes of these differences are very easily 

 explained, when the variations of character in the unaltered 

 part of the bed are considered. Some of the columns thus 

 consist of sandstone highly indurated, while others are formed 

 of the same indurated shale as that of Arran already described, 

 and might thus with the same propriety be called columnar 

 ironstone. In some places the columns consist of jasper ; that 

 is to say, the argillaceous sandstone is here indurated to such 

 a degree, as to put on the appearance and characters of that 

 rock. It is thus plain, that the term columnar jasper has 

 been improperly applied to this whole mass of prisms, as the 

 occurrence of this particular substance is but partial. It is 

 indeed so irregular as not even to exist necessarily throughout 

 the whole of any one column, or a single column is not always 

 formed of one substance ; as for example, of jasper, of ferru- 

 ginous hard shale, or ironstone, or of indurated sandstone. On 

 the contrary, the jasper often forms a very small part of one 

 column, and all these substances are sometimes found inter- 

 mixed in the same. This is a circumstance, therefore, which 

 may easily be understood from considering, as already hinted, 

 the mixed and irregular nature of the sandstone bed where it 

 retains its natural form and disposition. 



It is now necessary to inquire respecting the probable cause 

 to which these prismatic concretions owe their forms and origin. 

 It is no assumption, if a fact, to use this language. If there is 

 anything certain in the whole range of geological science, it is 

 that the secondary strata have been deposited from water, at 

 least essentially, in their present predominant forms ; and 

 from this cause they assume that simple stratified appearance 

 which is the natural result of this process, and on which it is 

 quite unnecessary to dwell. But, in many places, they are 

 partially modified, or present appearances of a limited nature, 

 inconsistent with their prevailing characters, and probably, 

 therefore, the consequences of actions of a different and local 

 nature, simultaneous with, or posterior to their deposition. 



The structure in question must be ranked with these, as 

 no depositions of earths from water, under the usual circum- 



