80 On an Arenaceo-calcareous Substance. 



These terraces stand at various altitudes above the present bed 

 of the river ; according to the different periods of time at which the 

 water, once effecting its descent to a lower stage, abandoned the 

 surface on which it had last flowed. At the point here to be 

 described, they appear to reach to about sixty feet 



At this place, as in most others, they present an aggregation of 

 rolled pebbles of various sizes, accompanied by a few angular 

 fragments, which seem to have undergone a less distant trans- 

 portation, and succeeded by coarse gravel and siliceous sand ; 

 the larger materials being, as usual, predominant towards the 

 bottom, and the smaller at the top. As the river is at present 

 impelled against the foot of this bank, it exhibits a recent section 

 resulting from the constant waste it experiences ; the surface of 

 the declivity being frequently renewed ' 'y the losses which the mass 

 undergoes from the occasionally increased state of the stream. 



The upper and flat surface of this bank is an extensive and cul- 

 tivated plain, nor is any rock to be seen for a considerable space ; 

 the alluvial soil covering the subjacent strata in most places to the 

 depth already described, and the river not having as yet reached 

 them any where in the immediate vicinity of the spot in question. 



The fundamental rock, thus far below the site of the present 

 appearance, is the red sandstone which succeeds the primary 

 country of the Highlands at Birnam, and extends, with the excep- 

 tion of trap and its congenerous rocks only, far to the southward. 

 There is no appearance of limestone in the vicinity, nor are any 

 fragments of this substance obvious among the transported 

 materials, although there is little reason to doubt that such exist 

 in the soil. 



In examining the sandy bank, thin and indurated lamia ec are 

 seen interposed f.mong the loose materials, protruding to a small 

 distance, and, in consequence of their superior tenacity, resisting 

 the action of the stream and that of their own weight. On di- 

 vesting them of the loose sand in which they are enveloped, they 

 are found to present a great variety of stalactitic forms, generally 

 more or less complicated, and often exceedingly intricate and 

 strange. The two simplest modifications that occur, may be con- 



