TRANSACTIONS OF THE SECTIONS. 55 



from the nortliem extremity it becomes a mere passage for the surplus watei", 

 about eighty yards wide, terminating in a narrow opening to the sea, liable to be 

 choked up. It is separated from the sea by a sand-bank, at fii'st 900 yai-ds -v^ide, 

 diminishing to 300 yards. This is generally high enough to keep out the highest 

 storm- waters ; but there are remains of two old cuts thi'ough which the sea enters 

 occasionally. The area of the lagoon is 4800 acres ; the depth averages live or six 

 feet below the mean level of the Mediterranean. Besides a large number of winter 

 torrents, there is one river emptying into the lagoon. The area supplying the tor- 

 rents is about 20,000 acres, and "that feeding the river and torrents to the north of 

 it, about 25,000 acres. The mean annual rainfall of the district is estimated 

 (from three years' observations) at twenty-four inches, of which ten inches fall in 

 October and 'November, which is the rainy season. 



The history of the formation of the lagoon is very clear. About 2000 years ago 

 the bank of sand now nearly closing it did not exist. The bank must have com- 

 menced and increased gradually till about 300 years ago, when it was completed. 

 The gi-adual depopidation continued for about two centuries. The bank is about 

 seven miles long, a quarter of a mile wide, and fifteen feet high above the shallow 

 sea-bottom outside. 



There is proof of the recent closing up of the lagoon in old walls and fragments 

 of buildings near the northern end. There is no doubt of the comparatively 

 modern elevation of all Corsica, but this will not account for the lagoons. These 

 are due to the drifting of the sands, as explained ; and as the mihealthiness of the 

 island (which is extreme) is due to the lagoons, it would be diminished if they 

 were greatly reduced. The author believes that by separating the drainage areas 

 of the lake into two parts, and removing, by pmuping, the whole southern part, 

 which is perfectly practicable, at least 4000 acres of rich land would be recovered, 

 and the rest of the land rendered cultivable. The operation could be adopted 

 with great facility and at small expense, and could not fail to exercise an impor- 

 tant iuilueuce on the material prosperity of Corsica. 



Oil the Granites and other Roclcs of Ben More, from a Letter addressed to Pro- 

 fessor Phillips. Bi/ His Geace the Duke of Aegyll, LL.D., F.B.S. 



When I was in the island of Mull the other day, I observed a fact wliich may 

 perhaps be of some interest, which is, that Ben Craig, one of the lower shoulders of 

 Ben More, exhibits very clearly the passage of a rock which looks like pure trap 

 into reo-ular gi-anite. At the base of the shoulder moimtain, which may be about 

 2000 feet high, it is a mass of a fine-grained compact granite. At the top it is a 

 mass of stuft" which weathers white, and has a fracture like some kinds of trap. At 

 an intermediate elevation the trap-like stuff contains many crystals of felspar very 

 distinctly separated. A little lower down these crystals become more frequent, and 

 a granitiform rock appears ; and very little lower the regular granite suhtervenes. 

 I could detect no separation. The top of the mountain is very white, the rock 

 very shattered, some of it very light, with one or two dykes passing through this 

 trap-like mass. The dykes are of a closer texture, v.'ith white crystals, wholly 

 unlike the surroimding mass. I must add that, though this stuff breaks like a 

 kind of trap, it is wholly unlike trap in other respects. It is perfectly amorphous, 

 both in structure and iu'the mode in v>rhich it occurs. It is not laid in sheets and 

 terraces like the traps of the same island elsewhere. In short it is not trap at aU, 

 but the matter out of which granite seems to have been made by pressure, and 

 crystallization under pressm-e. 



I send in a separate cover — 1. The gTanite as it appears at the base of the hill, 

 or two-thu-ds of the way up ; 2, a bit showiug the appearance of the felspar crys- 

 tals where they appear ; and 3, the rock at the top, of which a vast mass of the 

 mountain is composed. The whole structure of Ben More in SIull is fidl of inter- 

 est. The summit peak is of stratified rock, mica-slate ; and aU the lower shoulders 

 are granite or igneous rock becomhif/ granite. 



