746 R. MALLET ON AN HITHERTO UNNOTICED CIRCUMSTANCE 



the axis, through ducts situated at very great depths, no falling-in 

 or alteration of surface may be apparent, just as the filling-up of 

 cut-out beds of coal by the " creep " or gradual forcing-up by super- 

 incumbent pressure of the " coal seat" until that reaches the roof, 

 gives no evidence at the surface of such subterranean movements ; 

 but where the excavation goes on at bat an inconsiderable depth, 

 portions of volcanic cones give way and fall, so that the descending 

 material more or less fills up the previous cavit}\ This I consider to 

 be the mode of production of the Val del Bove, and of the singular 

 wall-sided cavity known as the Cisterna, and situated upon the 

 Piano above. Both these vast depressed and wall-sided cavities 

 appear to me to present many unmistakable evidences of having 

 been thus formed by dropping down, whether at one or at successive 

 times, of portions of the mountain, whose support beneath had been 

 eaten out until it became insufficient. The forms of the rocky steeps 

 surrounding these cavities, more especially that of the Cisterna, for- 

 bid the supposition that they were anywhere the flanks of ancient 

 craters ; on the contrary, they have all the characters presented by 

 rock masses which have been broken up and fallen from want of 

 support beneath. In my earthquake-expedition of 1857, I observed 

 in the great mountain gorge of Compostrina, which separates the 

 Val di Diano (where the river is called the Calore) from the next 

 valley lower down (where it assumes the name of Tanagro), a con- 

 siderable number of deep wall-sided cavities precisely resembling in 

 physical character that of the Cisterna — though not volcanic, yet pro- 

 duced by a mechanism closely analogous in its effects. 



The water of the Calore debouches from the Yal di Diano by two 

 channels diverging from the same spot : one of these is a subaerial 

 torrent, as muddy as all the rest of the river, which finds its way 

 through the tremendous gorge into the lower valley ; but another, 

 subterraneous channel, into which a prodigious volume of water 

 enters, finds its way in darkness beneath the solid limestone rock of 

 the mountain, and only reappears at the mouth of the great cavern 

 called St. Michael, where the water, now become perfectly pellucid 

 by subterranean deposition, reappears and joins the muddy stream 

 of the subaerial torrent. The wall-sided cavities just referred to 

 are seen to form an irregular chain on the flank of the mountain, 

 and are no doubt situated vertically above the irregular duct below, 

 which, continually eroding its whole walls and weakening its 

 covering rock, has in some places caused the latter to drop down. 

 Some phenomena well known to the inhabitants definitely support 

 this view of the formation of these vast pits with wall sides, and a 

 bottom covered with huge fragments of broken-up rock. On more 

 than one occasion the water discharging from St. Michael has sud- 

 denly become turbid and for a time greatly diminished in volume ; 

 and on such occasions a new cavity has been found to have formed 

 somewhere on the mountain. 



It is only necessary to examine the forms of these great pits, so 

 greatly resembling the Cisterna and those to be found in many 

 places on volcanic mountains, to come to the conclusion that both 



