1873.] GA8TALDI GLACIEE-EKOSION W ALPINE VALLEYS. 397 



in calc-schist, and others in much harder rocks, as for example in 

 felspathic, amphibolitic, and chloritic schists, &c. These are nar- 

 rower and less long, the others vaster and deeper. 



For my part I have no doubt that these high cirques are the beds 

 formerly occupied by glaciers but little anterior to the modern period. 

 Glaciers therefore are well able to excavate for themselves deep 

 beds in soft rocks, and also in rocks relatively hard, in high Alpine 

 regions. 



Let us now look at the plain, or rather at the mouths of the 

 valleys of the Alps. 



In general when entering our Alpine valleys from the plain, their 

 mouths are found to be narrow relatively to their length, their 

 width, and their orographical importance. 



Among all our valleys, that of the Dora Baltea (Ivrea and Aosta), 

 that of the Stura (Lanzo Balme), and that of the Biparia (Rivoli- 

 Susa-Cesanne), are distinguished in this respect. You know that 

 the valley of the Baltea, which is more than 100 kilometres long, is 

 only a kilometre wide at its opening. The valley of the Stura at its 

 outlet from the Alps at Lanzo opens on the plain by a regular door 

 entirely occupied by the torrent ; and the ascending road is obliged 

 to mount on the flanks of the valleys. A geological examination of 

 this locality leaves no doubt that it is the waters of the Stura which' 

 have excavated the gorge by which they now discharge themselves 

 on the plain. This work of erosion is still going on; and, in fact, 

 under the bridge we see a gigantic pot-hole, from 6 to 8 metres in 

 diameter, in process of formation. The valley of the Riparia is more 

 than 90 kilometres long from the Col du Grand Miol to the point 

 where it opens on the plain of the Po at St. Ambrogio, and has 

 many lateral valleys — that of Novalesa, which ascends to the 

 plateau of Mont Cenis, that of Bardoneche, comprising the Roches- 

 molles, the Frejus, the valley of Meleget and Tabor, the valley of 

 Mont Genevre, that of Thures, and the long valley of Sauze de 

 Cesanne. Yet this valley of the Riparia, so remarkable for its 

 length and interior breadth, is only 800 or 900 metres wide at its 

 exit at St. Ambrogio. What is the cause of this remarkable fact ? 



The experience which I have acquired in many years' study of the 

 Alps, from Ticino on one side to the Apennines on the other, has 

 convinced me that all the calcareous and felspathic rocks, the 

 granites, porphyries, gneiss, &c, are easily disintegrated by atmo- 

 spheric action and the power of water, whether liquid or solid, 

 giving rise by decomposition, degradation, and fracture to an enor- 

 mous quantity of detritus, mostly of very small size. On the con- 

 trary, the rocks which best resist these agents are the amphibolites, 

 diorites, syenites, amphibolitic schists, euphotides, serpentines, &c. 



These rocks, when present in colossal masses, are, it is true, 

 subject to great landslips, formed of large blocks which do not lose 

 their angularity, nor do they form regular deposits, and therefore 

 never give rise to the formation of good soils ; in other words, they 

 permanently resist atmospheric action. For this reason the most 

 difficult summits and the boldest points abruptly rising into the air 



VOL. XXIX. PAET I. 2D 



