368 ON THE ALLUVIAL DEPOSITS. 



from lost strata. Thus have I at length unexpectedly 

 explained the trap boulders, bearing every appearance 

 of transportation, which are so abundant and trouble- 

 some in Scotland ; having found them, with all the 

 polish which friction could give, imbedded in the un- 

 moved clay derived from the original rock. It is easy 

 also to see, how further deceptions may arise in this 

 case : as a river may thus transport the rolled stones 

 of a conglomerate, themselves transported, ages before, 

 and rounded by far other rivers. Such alluvia occur 

 in the Alps, and also in Scotland ; while, probably, 

 many examples in England, otherwise explained, will 

 admit of the same reasoning. 



Alluvia of Descent. 



The necessity of entirely revising the alluvia,, hi- 

 therto confounded by geologists, to the utter confusion 

 of the science and the production of the most extra- 

 ordinary hypotheses, has caused me to give this name 

 to those which are produced by a combination of gra- 

 vity, aided by rain, with the ordinary disintegration of 

 the summits of mountains. These occur on all decli- 

 vities, and consist of clay and sand with fragments, often 

 of great size, which, if generally angular, are sometimes 

 slightly rounded, from partial attrition or decomposi- 

 tion. Their real origin is indicated by their position, 

 by tracing the progress of the operation, and by the 

 nature of the substances. Their depths vary according 

 to innumerable circumstances, and they often descend 

 so far as to occupy the vallies beneath. Thus, it is not 

 exclusively by the action of rivers, that the mountains 

 are lowered, as I formerly remarked; less perceptible 

 agents give their aid, in an effectual, if in a silent manner. 

 In rocky mountains with considerable acclivities, the 

 protrusion of the naked summits, with the visible fall 



