Geological Society. 541 



tircly of enormous masses of granite. This promontory, he con- 

 ceives, was once a lateral moraine, and as it projects nearly half a 

 mile beyond the extremity of the glacier, and is covered with old 

 trees, he infers that the glacier has diminished in length to that ex- 

 tent. 



Mr. Darwin says it is impossible to explain the distribution of 

 boulders without the agency of ice, but he adds, that neither the till 

 of the Strait of Magellan which passes into, and is irregularly inter- 

 stratified with, a laminated sandstone containing marine remains, 

 nor the stratified gravel of Chiloe, can have been produced like 

 ordinary moraines. The boulders, likewise, on the lower levels at 

 the head of the Santa Cruz river, he considers, could not have been 

 distributed in their present position by glaciers, the surface having 

 been modelled by the action of the sea ; and the little inclination of 

 the high plain from the ridge of the Cordillera to where the boulders 

 occur, as well as the absence of mounds or ridges on it, and the form 

 of the fragments, render it very improbable that they were pro- 

 pelled from the mountains by ancient glaciers. Hence, he con- 

 cludes, that the blocks of Tierra del Fuego and Chiloe were certainly 

 transported by floating ice, and most probably those of the low and 

 high plains of Santa Cruz. Finally, he is of opinion, from the ge- 

 neral angularity of the blocks, and from the present nature of the 

 climate of the southern parts of America, which favours the descent 

 of glaciers to the sea in latitudes extraordinary low, that it is more 

 probable that the boulders were transported on the surface of ice- 

 bergs, detached from glaciers on the coast, than imbedded in masses 

 of ice, produced by the freezing of the sea. 



May 19th. — A paper '.' On the Agency of Land Snails in corroding 

 and making deep Excavations in compact Limestone Rocks," by the 

 Rev. Professor Buckland, D.D., F.G.S., was first read. 



During the meeting of the Geological Society of France at Bou- 

 logne, in September 1839, Dr. Buckland's attention was called by 

 Mr. Greenough to a congeries of peculiar hollows on the under sur- 

 face of a ledge of carboniferous limestone rocks. They resembled 

 at first sight the excavations made by Pholades, but as he found in 

 them a large number of the shells of Helix aspersa, he inferred that 

 the cavities had been formed by snails, and that probably many ge- 

 nerations had contributed to produce them*. 



A few years since, the Rev. N. Stapleton informed the author that 

 he had discovered at Tenby, in the carboniferous limestone on which 

 the ruins of the castle stand, perforations of Pholades 30 or 40 feet 

 above high- water level ; but having recently examined the spot, Dr. 

 Buckland ascertained that these excavations were the work of the 

 same species of Helix as that which had formed the cavities in the 

 limestone near Boulogne, and he found within them specimens of 

 the dead shells as well as of the living. The mode of operation by 

 which the excavations were made, he conceives, is the same as that 

 by which the common limpet {Patella vulgata) corrodes a socket in 



* See Bulletin Geol. Soc. France, vol. x. p. 434, 1839. 



