96 REPORT—1862. 
difference in the state of the carbonate of lime; and that, other conditions being 
the same, shells which were composed of calcite are preserved, whereas those 
composed of arragonite have been altered. This appears to depend on the fact of 
the particles of arragonite being in a state of unstable equilibrium. When prepared 
artificially, it has a great tendency to pass into calcite; and if this change took 
place in shells, their organic structure would be very apt to be destroyed, though 
the shell might remain as a crystalline mass of calcite. If, however, the cireum- 
stances of the case were such that the calcite formed at the expense of the arrago— 
nite of the shells had a greater tendency to crystallize elsewhere rather than in 
situ, they would be removed, and leave more or less perfect casts. On the con- 
trary, calcite having no such tendency to change, shells composed of it might, 
under similar conditions, remain nearly in their original state. 
On the Comparative Structure of Artificial and Natural Igneous Rocks. 
By H. C. Sorsy, F.B.S., Fe. 
As is well known, Sir James Hall and Gregory Watt, by fusing and slowly cool- 
ing basalt, obtained a stony mass, to a certain extent similar to the original rock. 
Various writers on the subject have since contended that the product is not, like 
the original, composed of several distinct minerals, but made up of only one kind 
of crystals. The author, however, showed that, when thin transparent sections are 
examined with a high magnifying power, it may be seen that the artificial rock is 
really an aggregate of the three principal minerals of the original basalt, which, 
nevertheless, are developed and arranged in such a very different manner that it is 
easy to understand why this fact has been overlooked. Indeed, the difference in 
general structure is so considerable that, probably, other causes besides a slower 
cooling were instrumental in producing the peculiar characters of the natural rocks. 
On Scutes of the Labyrinthodon, from the Keuper Bone-Breccia of Pendock, 
Worcestershire. By the Rev. W. 8. Srmoyvs, M.A., FU.GAS. 
The remains of this Triassic reptile have been found in the Keuper sandstone of 
Warwick and Leamington, but had not hitherto been detected in the Trias of 
Worcestershire or Gloucestershire. The scutes and bones found by Mr. Symonds 
were submitted "to Prof. Huxley. They occur chiefly in the “ bone-breccia,” de- 
scribed by Mr. Symonds in the ‘Transactions of the Geological Society,’ and are 
associated with numerous spines of fishes. 
On the Geology of a Part of Sligo, By A. B. Wrxnu, F.GS. 
In this paper the author stated that he had put together a few notes upon an 
extensive district. They were made during a short tour to the co. Sligo and part 
of Leitrim, in the summer of 1862; and he alluded to papers by Sir R. Griffith, 
Archdeacon Verschoyle, and Mr. John Kelly, in the ‘ Proceedings of the Geological 
Society,’ all of which referred to the country under consideration. He then pro- 
ceeded to describe the district as composed of a widely spread, nearly horizontal series 
of stratified rocks, consisting of sandstones below and above, with a thick band of 
limestones interstratified with other sandstones between. This horizontal group 
represents the Carboniferous formation, from the Millstone-grit downwards, and 
probably a part of the underlying Old Red Sandstone ; and the thickness of the 
group is little less than 2000 feet, roughly estimated from the heights of the 
mountains formed by these rocks, Cutting across the country formed by these hori- 
zontal beds is the rugged chain of the Ox Mountains, extending from Mayo into 
the co. Leitrim. Some of the most picturesque valleys in the district, like that of 
Lough Gill, are the lateral ones along the flanks of the Ox chain, which, being 
formed of gneissose, micaceous, and quartzose rocks, have a totally different aspect 
from the mountains formed of the limestone and other horizontal beds. The ser- 
pentine garnet rock and trap-rocks of these older mountains were next alluded to, 
and it was stated that, although they seemed to occupy fissures running in various 
opposite directions, their master-joints or divisional planes were nearly parallel. 
The denudation which exposed the Ox Mountains, and removed the thick series of 
