262 Geological Society. 



feet, sloping inwards 11°, beds of sandstone in a sea- worn cave, 

 proving at least one other disturbance in addition. Subsequent to 

 these great disturbing changes, there occurred a series of elevations 

 and depressions, indicated by mixed beaches and sea-bottoms at dif- 

 ferent levels and by the surface of the rock perforated by Lithodomi 

 and sea-worn to the very summit, indicating that the amount of 

 change of level in these comparatively modern times — for the fossils 

 in these deposits are in every case identical with species now living 

 in the neighbouring seas — exceeded the height of the mountain, or 

 1470 feet. There are evidences, also, of a series of movements of 

 depression. All these changes must have preceded the historical 

 period, as previous to the last change, Gibraltar must have been an 

 island, of which there is no record ; the most ancient accounts de- 

 scribing it as it is now. The upheaving forces must have been deep- 

 seated, as there are no erupted igneous rocks near. 



December 4. — A paper was read, entitled, " Remarks on the Geo- 

 logy of British Guiana." By the Chevalier Robert H. Schomburgk. 



The geology of the district of British Guiana is chiefly confined to 

 primitive rocks. At the mouth of the Orinoco is an extensive delta 

 consisting of blue clay, which, when pierced, gives a supply of water, 

 and Artesian wells have been sunk here in many places with success. 

 Below the clay appear the remains of an ancient forest. The allu- 

 vial flat is terminated by sand-hills, beyond which occurs granite in- 

 tersected by numerous greenstone dykes, and then commence the 

 savannahs, which are traversed by large beds of conglomerate often 

 containing iron ore, and pierced with lofty porphyritic hills. The 

 savannahs are supposed to be the bed of an ancient lake. A region 

 in which much jasper occurs next succeeds, and then a remarkable 

 range of granitic mountains ; and the author directed especial atten- 

 tion to the insulated rocks of grotesque form abounding in the di- 

 strict. He also remarked on the probability of gold being found in 

 the river-courses, and on the appearance of the well-known diamond- 

 matrix of Brazil. 



A letter was next read from Mr. Trevelyan, remarking on the oc- 

 currence of polished and scratched surfaces in the neighbourhood of 

 Conway, on the ascent of Moel Siabod, from CapelCarig, on Snow- 

 don, and in other localities in North Wales. 



December 18. — A paper was read " On the Pipes or Sandgalls in 

 the Chalk and Chalk -rubble of Norfolk." By Joshua Trimmer, Esq. 



The observations recorded in the present paper were made in chalk 

 pits near Norwich, and the surface of the chalk was observed to be 

 furrowed by irregular cavities, or deep cylindrical conical pipes, en- 

 tering the chalk from the channeled surface.* The contents of the 

 furrows appeared to be fine sand mixed with a light- coloured amber 

 or yellow ochre, the former often filling up the cavities both here 

 and elsewhere, llie author considers, that although chemical agency 

 may have assisted in the formation of these cavities and pipes, yet 

 that it is necessary to admit also mechanical action, and he refers to 

 several instances of the known effects of rain-water on cliffs, and ex- 



• Some remarks on this subject by Prof. Ehrenburg will be found in the 

 Annals of Natural History, vol. ii. p. 161. 



