TRANSPORTED STONES. 



313 



By what agents were these beds raised to their present elevation ^} 

 Satisfactory solutions to all these enquiries will probably long rennain 

 desiderata in geology, though, in some instances, we can now arrive 

 at a high degree of probability, by referring to causes in present ope- 

 ration. These scattered fragments or masses of rock, with beds of 

 loose stones and gravel, or of superficial sand or clay, are comprised 

 by French geologists under the appropriate name of terrains de 

 transport ; a name, however, which cannot well be introduced into 

 our language. We shall, therefore, divide them into three groups, 

 adopting the names generally received. Scattered blocks of rock ; 

 diluvial beds or diluvium ; and alluvial beds or alluvium ; using the 

 two latter without any reference to theory. Alluvial beds, consist of 

 the sand, soil, or stones, brought down by rivers, and deposited in 

 their beds, or scattered upon their banks, or carried into the sea or 

 into lakes, forming deltas at the mouths of rivers. Diluvium, or 

 diluvial beds, comprise both the scattered blocks of rock, and the 

 beds of stone or gravel, that are carried into distant districts. They 

 are called diluvial, on the supposition that they were transported dur- 

 ing some great convulsion, by deluges or inundations ; or, in other 

 words that they were removed by causes more powerful than any 

 which are seen in constant operation. 



In order to form a more distinct idea of the causes which have 

 transported the beds and fragments of stone into their present situa- 

 tion, we shall first consider the causes that are daily wearing down 

 the loftiest mountains and cliffs, or undermining the solid ground on 

 the sea shore. The disintegration of rocks and mountains is constant- 

 ly taking place, by the incessant operation of atmospheric causes. 

 The infiltration of water into the fissures of rocks, and its expansion 

 by frost, often produces sudden falls of immense masses of rock. 

 The slow operation of descending currents, excavates the soft beds 

 in the lower parts of mountains; and the upper rocks, being under- 

 mined, fall, with a tremendous crash, into the vales below. Instan- 

 ces of this kind have occurred in our own times. By both these 

 causes, the process of disintegration is rapidly going on in the Alps ; 

 but such is the immensity of these enormous mountain ranges, that 

 ages pass away, before any diminution of their bulk is perceived. 



In Alpine districts of great elevation, there is also another cause, 

 more exposed to observation, which is ever in action during the sum- 

 mer months. The snow upon the mountains below the line of eter- 

 nal congelation, when it begins to dissolve, forms numerous rivulets, 

 that unite into large streams, and descend in cataracts with impetu- 

 « ous force, excavating deep ravines in the lower rocks. To use the 

 words of Professor Playfair, they are " Nature's saws, incessantly at 

 work, cutting down the mountains." 



The vignette in the titlepage of this volume represents the upper 

 part of the valley of Sext, in Savoy, in which the water, descending 

 from the Alpine snow on the Buet and other mountains, is' seen rush- 



40 



