by the passage over them of Sharp Detrital Matter. 115 



able to the parent rock, which, however distant, is always at 

 a higher level than themselves ; that they often rest upon 

 beds either secondary or alluvial ; and, lastly, that the upper 

 surface of rocks protected by soil is, in many cases, so fur- 

 rowed as to resemble a wet road along which a number of 

 heavy and irregular bodies have been dragged, those fur- 

 rows generally agreeing, in parallelism, both with one an- 

 other, and with the ridges and large features of the dis- 

 trict;" 



Mr Greenough, not considering the force of running water, 

 either as exhibited in the torrents of rivers or in the sea, 

 sufficiently great to account for such results, ascribes them 

 to the deluge. Hutton had ascribed them to the tumultuous 

 rush of water consequent on great debacles, the result of 

 sudden elevations of mountain chains and masses. 



Dr M'Culloch, in his System of Geology (1831), observes 

 in his chapter on Changes in the Disposition of the Sea and 

 Land, " But I must not omit one argument supposed to 

 afford a strong evidence of such diluvial currents ; namely, 

 the scratches or marks of friction already noticed in rocks 

 where water does not now flow. Many of the quoted in- 

 stances occur in places where rivers have once run, under 

 the changes already pointed out ; while if the others confirm 

 the former existence of currents that could not well have 

 been rivers, they are not competent to prove such movements 

 of water as I have here rejected (diluvial). I shall inquire 

 further of them presently, as the probable effects of heavy 

 alluvia transported by water under other causes," an idea 

 which is similar, if not identical, with that of Mr Mallet. 



Again, in his descriptions of alluvia, Dr M'Culloch has 

 one section denominated " alluvia of descent." He says, — 

 " The necessity of revising the alluvia has caused me to give 

 this name to those which are produced by a combination of 

 gravity, aided by rain, with the ordinary disintegration of 

 the summits of mountains. These occur in all declivities, 

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

 size, which, if generally angular, are sometimes slightly 

 rounded, from partial attrition or decomposition. Their real 

 origin is indicated by their position, by tracing the progress 



h2 



