BRITISH ASSOCLVrrON MEETING AT CAMBRIDGE. 



423 



deacon Versclioyle, and ]\ir. Jolm Kelly, upon parts of the same country, 

 and proceeded to describe the district as coin])osed of a j^reat nearly hori- 

 zontal series of stratified rocks, consistin<^ of sandstones below, carboni- 

 ferous limestones of impure, thin, and slialy character abov^e, and ])aler, 

 more pure limestones of the same group overlying them ; the whole sur- 

 mounted by 400 feet of the niillstone-grit series. This group of horizontal 

 rocks forms fine tabular mountains, with picturesque valleys, and a pecu- 

 liar likeness in all their profiles. Cutting across the country formed by 

 these horizontal beds is the rugged chain of the Ox Mountains, extending 

 from Mayo across the county of Sligo to the south of Lough Gill, and 

 terminating beyond Bcnbo mountain, in county Leitrim. Some of the 

 most picturesque valleys in the county are the lateral ones near this chain, 

 running along its flanks, and dividing the old micaceous and gneissosc rocks 

 of which it is formed from the horizontal series above alluded to ; and in 

 these valle^^s, as well as transversely crossing the chain, occur deposits of 

 serpentine of considerable size and interesting character. 



The denudation which exposed the Ox Mountains at the east end of the 

 chain was alluded to, and the circumstance stated, that the limestone con- 

 tained bands of sandstone which appeared to be of irregular thickness, and 

 were most numerous in the lower part of the carboniferous series seen in 

 the district under consideration. 



The occurrence of trap-dykes and mineral deposits was noticed, and 

 also the changes of level at recent periods, as well as the manner of the 

 occurrence of the drift, which was found, in one instance, to contain shells 

 of the common nmssel, at a considerable distance from the sea, and in cod- 

 nection with which, the horns, skulls, and other bones of fossil deer and 

 cattle had been found. The paper was illustrated, and a list of fossils 

 (by W. H. Baily, Esq.) found in the district was appended. 



OiS'" THE CAUSE OP THE DTFFEEENCE TN THE STATE OF PRE- 

 SERYATION OF DIFFEEENT KINDS OF FOSSIL SHELLS. 



By H. C. Sorby, F.R.S. 



Most geologists have, no doubt, remarked, that in very many rocks 

 certain kinds of fossil shells are well preserved, whilst others are very 

 badly preserved, or have entirely vanished, and left nothing but casts : 

 for instance, oysters retaining their organic structure, whilst many others, 

 like trigonise or most univalves, having lost their original structure, being 

 quite crystalline, or even having been entirely removed. After having 

 made many experiments with recent and fossil shells, the author has been 

 led to conclude that this difference was due to the original difference in the 

 mineral constitution of the various shells; and that when their carbonate 

 of lime was in the state of ralcite they were less prone to undergo any 

 change ; but where it was in the state of aracfomic they always have had 

 a tendency to become crystalline, by passing into calcite, or to be entirely 

 removed if the conditions were more favourable to the cr3'stallizatiou of 

 calcite in some other place rather than in situ. 



