220 : Reports and Proceedings. 
Pleurotomaria Youngiana from Craigenglen, Campsie, species both 
new to the Carboniferous rocks. 
The Cuarrman exhibited a suite of the Carboniferous fossils from 
Belgium (which he owed to the kindness of Professor De Koninck), 
and pointed out their relation to those of the Scotch Coal-measures. 
With great generic identity, there are slight specific differences, 
such as exist over similar areas at the present day. The philoso- 
phical problem involved, he said, is the connection between contem- 
poraneousness of formation and identity of species. It would appear 
that the probability of the contemporaneousness of identical species 
in any formation is in inverse proportion to the extent of area. 
Over limited areas nearly identical species were probably contempo- 
raneous ; but with extension of area identity of species becomes a 
valid argument against contemporaneousness. 
Mr. Joun SUTHERLAND read a paper On Silica, and its Mode of 
Formation in the Earth's Crust. After some introductory remarks, 
he said, that of all the components of the earth’s crust there was 
perhaps no other substance so universally present as silica, a com- 
pound of silicon and oxygen (Si O;). It may be called the bone or 
skeleton of the earth, giving hardness and compactness to all its 
covering. It is a principal constituent of granites, porphyries, clays, 
slates, &c. ; in its purest form it is found as sand, flint, quartz, and 
in more attractive forms as rock-crystals, opal, chalcedony, cairn- 
gorms, and many other minerals valued for their beauty. There are 
three varieties of silica: the crystallized, represented by quartz- 
crystals, with a specific gravity of 2°6; the so-called crystalline, re- 
presented by common flint, with the same specific gravity, and which 
is supposed by Rose to be an aggregation of crystals so small as to 
be undiscernible by the highest power of the microscope; and the 
amorphous, which is entirely devoid of crystalline structure, and has 
a specific gravity of only 2:3. Rose’s chief reason for supposing the 
crystalline variety to be built up of minute crystals is simply that 
it, like the first-named variety, polarizes light, and is unacted upon 
by alkaline solutions. Now, it is found that when silica of the 
specific gravity of 2°6 is subjected to fusion, its specific gravity is 
reduced to 2°3; and this fact has of late been made to occupy an im- 
portant place in discussions regarding the aqueous versus the plu- 
tonic origin of certain rocks. ‘The Plutonists assert, for instance, 
that granites are the result of fusion by heat; but on fusing it, its 
quartz or silica is reduced to the density of the amorphous variety 
obtained by precipitation from a solution, and besides will not crys- 
tallize. On the other hand, the latter process fails in producing 
silica of the specific gravity of 2°6. By subjecting the silica, how- 
ever, to a high temperature under great pressure of aqueous vapour, 
crystals having that specific gravity are obtained. Mr. Sutherland 
having stated some grounds for believing that crystallized silica may 
be produced in nature, without the aid of water, then referred to the 
subject of flints in the chalk-beds, and fossil shells filled with hard 
compact silica of the crystalline variety—a subject which had long 
puzzled geologists, and which has had hitherto no satisfactory solu- 
