194 Scrope — Origin of Valleys, 



A still more striking instance occurs in tlie celebrated earth- 

 pyramids of Botzen, in the Tyrol. Here, that a valley of very 

 large dimensions has unquestionably been excavated by subaerial 

 forces, chiefly by the direct descending force of rain, is proved by 

 the remnants left of the material originally filling the hollow — a 

 coarse gravelly conglomerate — in the shape of sugar-loaf-shaped 

 masses, some of them forty to sixty feet high, which have evidently 

 been preserved from destruction by the protecting covering of some 

 great boulder which still caps, or till recently capped, each separate 

 pyramid, and in the first case overhangs on all sides its precipitous 

 sides. Just in the same way the larger blocks on the surface of a 

 glacier have often acted as a shield or umbrella to the ice beneath 

 them, while the more exposed surface of the ice-field has been worn 

 down many feet lower by the rain-fall (See Forbes on the Glacier- 

 tables of the Alps. Travels, 1843, p. 25). 



These are some of the well-known examples of the denuding 

 agency of rain, in which it is impossible to attribute the result to any 

 other cause. But my chief object in this paper is to recal to the 

 attention of geologists who take an interest in this question, the very 

 remarkable and convincing evidence of the kind exhibited by the 

 valleys of Auvergne, into which, at various intervals during the pro- 

 cess of their excavation, streams of liquid lava have flowed from 

 neighbouring heights, flooding surfaces which must at the epoch of 

 each successive lava-flow have formed the lowest levels of the 

 valley. This evidence was brought forward by me in the first 

 edition of my Description of the Volcanoes of Auvergne, as far back as 

 1825, and the inferences I deduced from it confirmed and endorsed 

 from their own observations by Sir C. Lyell and Sir E. Murchison 

 a few years later.^ But I do not think due weight has been allowed 

 to it in the consideration of the controversialists on this disputed 

 question. Perhaps, therefore I may be pardoned for recalling their 

 attention to the crucial facts which this local coincid6nce of volcanic 

 and fluviatile or meteoric phenomena afford. 



" It is impossible," I wrote at that time, '' to observe the many 

 strips of the originally continuous fresh-water formation which rise 

 from the valley-plain of the Limagne in long tabular hills, transverse 

 to the general course of its main drain, the Eiver Allier, without being 

 convinced that each of these hills owes its preservation from the 

 destruction which has swept away the remainder of the formation to 

 its capping of basalt, which, by reason of its superior hardness, would 

 naturally protect the softer underlying strata from weathering, that 

 is to say, from the rain, frost, and other meteoric agents to which 

 the uncovered intervals of the marly plain left by the emptying of 

 the lake were exposed." " These plateaux of basalt are found at all 

 heights from 1,500 feet downwards above the water-channels of the 

 neighbouring vallej^s ; and some even of the most different in point 

 of level are situated geographically close to one another." 



"Take for instance the two neighbouring basaltic platforms of 



1 On the Excavation of certain Valleys in Auvergne. Phil. Mag. for April, 1829. 



