BRITISH ASSOCIATION MEETING AT CAMBRIDGE. 423 
deacon Yersclioyle, and ]Mr. Jolin Kell}^ upon parts of the same country, 
and proceeded to describe the district as composed of a great nearly hori- 
zontal series of stratified rocks, consisting of sandstones below, carboni- 
ferous limestones of impure, thin, and shaly character above, and paler, 
more pure limestones of the same group overlying them; the whole sur- 
mounted by 400 feet of the millstone-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 Benbo 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 gneissose rocks 
of which it is formed from the horizontal series above alluded to ; and in 
these valleys, as well as transversely crossing the chain, occur deposits of 
serpentine of considerable size and interesting character. 
The denudation which exposed the Ox ^lountains 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 irregiilar 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 mussel, at a considerable distance from the sea, and in con- 
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. 
01^ THE CAUSE OF TTTE DTFFERETfCE TX THE STATE OF PRE- 
SEEYATIOX OF DIFFEREXT KINDS OF FOSSIL SHELLS. 
By H. C. Soebt, 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 trigonia? 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 calcite they were less prone to undergo any 
change ; but where it was in the state of aragonite 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^stallization of 
calcite in some other place rather than in situ. 
