42 REPORT—1852. 
desirable to make a general examination of the principal diamonds in London, I went 
next day to the British Museum, and found there an interesting specimen, which 
threw some light on the yellow solid to which I have referred. This specimen was a 
piece of colourless diamond, uncut, and without any crystalline faces, about three or 
four tenths of an inch broad, and about the twelfth of an inch thick, and on its sur- 
face there lay a crystal of yellow diamond, with the four planes of semi-octahedron. 
This singular fact was illustrated by a large model placed beside it. Upon examining 
the original, I noticed a pretty large cavity in the thickness of the specimen, with the 
extremity of which the yellow octahedron was connected; and finding a portion of 
amorphous yellow diamond in the other end of the cavity, I had no doubt that the 
yellow crystal had emerged, in a fluid state, from the cavity when it was accidentally 
opened, and had immediately crystallized on the surface of cleavage. I am well 
aware that such an opinion makes a good demand upon the faith of the mineralogist ; 
but to those who have seen, as I have done, the contents of fluid cavities in crystal 
solidifying and even crystallizing on the face of cleavage, while another portion of 
the contents of the cavity escaped in gas—to those who have seen in topaz cavities 
numbers of regularly formed crystals, some of which, after being fused by heat, 
instantly recrystallized—the conclusion I have drawn will be stripped of much of its 
apparent extravagance. In examining a number of diamonds in the museum of the 
East India Company, to which Col. Sykes kindly obtained me access, and about forty 
or fifty in the possession of Messrs. Hunt and Roskill, I found many containing large 
and irregular cavities of the most fantastic shapes, and all of them surrounded with 
irregular patches of polarized light, of high tints, produced undoubtedly by a pressure 
from within the cavities, and modified by their form. Among these specimens I 
found one or two black diamonds, not black from a dark colouring matter, like that 
in smoky quartz, but black from the immense number of cavities which they con- 
tained. Tavernier has described a large and curious diamond which throws some 
light on the subject of this notice. It contained, in its very centre, a large black 
cavity. The diamond merchants refused to purchase it. At last a Dutchman bought 
it, and by cutting it in two, obtained two very fine diamonds. The black cavity 
through which he cut was found to contain ezght or nine carats of what Tavernier 
calls black vegetable mud! 

Geological Structure of the Counties of Down and Antrim. 
By James Bryce, jun., M.A., F.G.S. 
The author began by stating that the valley of the Lagan, on which the town of 
Belfast is situated, is a great depression on either side of which formations of dif- 
ferent ages are confluent. On the southern side of the valley the strata belong 
to the older formations; on the northern side they are the newest that occur in 
Ireland. Each of the counties of Down and Antrim is thus almost exclusively occu- 
pied with rocks peculiar to itself; those in the one county not including those in the 
other. The author then proceeded to describe the leading geological features of the 
County of Down. It contains two granitic tracts, which seem to have been elevated 
at different epochs. They are separated from one another, and each is wholly enclosed 
by a thick band of metamorphic slate, gneissose in its lower part, and passing upwards 
into flinty and common clay-slate. Superimposed conformably on these are other 
slates of a less crystalline type, whose aggregate thickness is enormous, and whose 
upper portions have yielded a few imperfect fossils, which seem to make them refer- 
able to the lower Silurian group; but as yet no definite lines have been made out 
to justify a classification. Over the slates, but unconformable to them, there occurs 
in the N.E. part of the county many strata abounding in fossils, which the author 
is inclined to refer to the Carboniferous and Permian systems. Among these no traces 
of coal have yet been found; which is remarkable, seeing that the system is otherwise 
so fully developed. 
The formations of Antrim were next described. These consist of triassic beds, lias, 
and the cretaceous system, including greensand and chalk; the whole overlaid by 
vast accumulations of igneous rocks presenting many varieties,—basalts, greenstones, 
greystones, porphyries, tufts, and ashes with lignites; which the author endeavoured 
to separate into distinct flows from certain foci of submarine volcanic action. Many 
new phenomena connected with the Giant’s Causeway were also described; and a 

