40 GEOLOGY OF THE CAMPSIE HILLS. 



from some other cause. This last-mentioned class, 

 do not, however, (as is generally thought,) hold 

 all the formations of the crust of our globe, of 

 which we can claim acquaintance, to be of igneous 

 origin : they admit clearly that some of the strata 

 have been formed by a deposition from water ; 

 but for the consolidation of these strata so formed, 

 they think it necessary to call in the aid of inter- 

 nal heat, and hold these polygonal forms to have 

 been produced in consequence of its effects. Such, 

 upon this point, is nearly the general outline of 

 the difference of opinion subsisting between these 

 two parties. But it ought to be remembered, that 

 nature frequently produces the same effects by 

 very different means, and that the columnar con- 

 figurations are formed not only in the vioist but 

 also in the dry way ; which can be here easily ex- 

 emplified. The aqueous origin of these forms, 

 may be observed upon any extended surface of 

 clay or of mud, which has been the deposite of 

 water, and where evaporation has taken > place. 

 In such situations, when evaporation has taken 

 place to a considerable degree of dryness, the de- 

 posited soil gradually shrinks, cracks in various 

 directions, and takes polygonal forms. If the soil 

 in such situations is shallow, the tabular form is 

 exhibited ; but if the soil is of any considerable 

 thickness, the columnar form appears. Beautiful 

 displays of these forms on a small scale, may fre- 



