260 Dr. Mac Culloch on a 



original internal corresponding structure, although that is not 

 visible in the fresh rock, perhaps in consequence of the nature 

 and uniformity of the colour. Having, in another communi- 

 cation, stated the facts and arguments on both sides respecting 

 this process in basalts, I need not notice it further here. But 

 it is plain in the case of this sandstone, that any exfoliation 

 which the prisms may eventually undergo will not be the con- 

 sequence of superficial decomposition from the action of the 

 air, but will have its foundation in a peculiar concretionary 

 structure of a laminar nature concentric to the axes or circum- 

 ferences of the prisms. 



Another remarkable variety of internal concretionary struc- 

 ture also occurs here (see fig. 2). In this a minute columnar 

 tendency within the principal column is indicated in the 

 same manner by variations of colour. Whether this is ulti- 

 mately to become actually divided into small prisms cannot 

 be conjectured ; but no marks of such division are as yet to be 

 seen. But I must remark that this case also presents an exact 

 analogy to one occurring in a trap rock (hard claystone) in 

 Morven, where a similar large division of the rock is sub- 

 divided into prisms of half an inch, less and more, in diameter, 

 which are capable of being removed to the depth of two or 

 three inches. A parallel case occurs also at Strontian. It is, 

 lastly, necessary to describe the variations of mineral character 

 which this bed of sandstone undergoes, when it will be seen 

 what the true nature of these disputed columns is. 



The bed, in its ordinary state, is, as already remarked, the 

 common red sandstone of the country, and consists, either of 

 red quartz sand alone, or of quartz mixed with red clay, or 

 with calcareous earth, or both. It is often so charged with 

 clay, as almost imperceptibly to pass into a red shale. In the 

 transition, there are, of course, many arenaceous, scarcely de- 

 finable substances, of an intermediate nature. Although the 

 general mass of the bed is red, there are white portions inter- 

 mixed ; but these are small in quantity when compared to the 

 red part of the rock. 



It is very perceptible that the greater part of this bed, where 

 it is here accessible, is harder than this rock generally is found 

 to be in this neighbourhood. But it becomes decidedly more 



