372 Dr. Mac Culloch on the Parallel Roads of Glen Roy. 



mutation of matter by degrees forms a plain or a delta at its mouth, 

 with a level surface ; the constant action of the waves continuing to 

 level the accumulated materials until the growth of peat or soil from 

 the ordinary decomposition of vegetables, increases its elevation to 

 such a degree, as to exclude the further access of the waters. In- 

 stances of such deltas are too common to need mention, and they 

 are found at the entrances of all rivers whether lateral or direct ; 

 their extent and form being proportioned to the various conditions 

 under which the different streams have entered, and which must 

 already be too obvious to require enumeration. If a lake under such 

 circumstances were drained, these deltas would be found to termi- 

 nate in gradual slopes, varying according to the inclination on which 

 they were deposited. 



Let us now compare such deltas with the present terraces of Glen 

 Roy ; that part of its structure which appears at first sight uncon- 

 nected with the previous existence and actions of a lake. Assuming 

 the imagined lake to have stood at the level of the lowest line, or 

 its third elevation, it is easy to understand that the present terraces 

 which exist at and near that level at the upper extremity of the 

 valley, are the deltas which appertained to the Roy ; while the 

 lateral ones, wherever they are found, are the remains of similar 

 deposits accompanying the lateral streams that still exist. To ex- 

 plain the present abrupt declivities of these terraces we must have 

 recourse to those actions which have succeeded to the complete 

 drainage of the valley, and which are precisely those already noticed 

 in a former argument, where a shifting river wanders over an allu- 

 vial strath. In consequence of the present passage of the Roy, these 

 deltas have been worn down at their bases, by a succession of changes 

 of which the marks are every where seen in the minor terraces al- 

 ready described, that accompany its course along the valley. Hence 



