2G4 
THE GEOLOGIST. 
in the Hartz, where, it is well known, many minerals are found in which selenium 
and some other elements are combined, such as seleniuret of lead, of silver, of 
mercury, and of mercury and lead. There are also several other seleniurets in 
that locality —Yours, &c., James R. Gregory. 
Fracture or Tlint-Pebbles, — Deah Sir, — In reference to the fracture 
of flint-pebbles from Charlton, p. 73, I would observe that the bed, No. 1, 
from which they were taken may be considered as having been drifted, 
from the confused manner in which the pebbles and sand are heaped 
together ; indeed, we may infer, from the resemblance both of the sand and 
pebbles to No. 2, that they were derived from that bed, probably at some dis- 
tance off, where it appeared at the surface ; and which after being denuded was 
again deposited and strewn over a large area, part being the locality 
where our section was taken, and some distance above its true position, which 
would be No. 2. We may presume (if this took place during the drift-period) 
that the pebbles were for some time surrounded and suspended by ice, long 
enough to cause the water they contained, however minute its quantity, to 
become thoroughly frozen, which, when the mass was thawed, would cause 
the flints to be traversed by numerous imperceptible fissures. These stones, 
imbedded in the sands, would hold together, but when extracted, a "tap" 
with the hammer proves their existence in the shattering of the flint. 
In all the specimens broken nearly the whole of the fragments assumed a defi- 
nite form. The concave and convex sides of each fitting on to the convex 
and concave sides of its neighbour; and so perfect is this arrangement, that 
with a little trouble the pebble may be put together by replacing the fragments; 
and if held firmly in the band will exhibit scarcely any traces of the numerous 
cracks. — Yours truly, Edmund Jones. 
Crenate of Ammonia in an Oligist of Devonian Age. — Dr. Phipson 
has communicated a note to the Comptes Rendus for May, on the occurrence of 
an organic matter in an oligist of Devonian age in Belgium. Its oolitic struc- 
ture caused him to think that it had been formed by incrustation in marshy 
tracts of the eggs of aquatic iasccts, as in the case he described in a former 
volume of this magazine, of the oolitic limestone in the great lakes of Mexico ; 
and it was this character which, in spite of the antiquity of the rock, lead him 
now to examine t.iis oligist to find if it had any any organic matter that could 
be regarded as the debris of plants or of aquatic insects. 
The result of this chemical analysis was that the oligist was found to con- 
tain more than four per cent, of crenate of ammonia, an azotized organic salt 
discovered by Berzelius, and the products of the decomposition of vegetable 
and animal matters floating in water. The mineral contained, moreover, traces 
of phosphoric acid, whence it is very probable it was formed like the modern 
ochres, and that it may owe its oolitic structure to the eggs of aquatic insects. 
Variation of Temperature Exhibited in Different Geologic Eras. — 
Sir, — Are there any evidences of similar alterations of general terrestrial 
climate in any of the older geologic periods, such as occurred in the Glacial 
epoch, namely, periodic alterations of hotter and colder general climatal condi- 
tions ? Your kind reply would oblige a young student who is much interested 
in the subject of former climatal changes. — E. Weld, Doncaster. 
This matter has jusc been very nicely treated in Mr. Page's new book "Life 
on the Earth," where he gives a diagram of undulations in which the 
Silurian, Carboniferous, Oolitic, Tertiary, and Recent Eras are made to repre- 
sent the cycles of warmer temperature ; and the Cambrian, Old Red, Permian, 
Chalk, and Boulder drift the alternate colder periods. This idea of colder and 
warmer cycles as affecting the northern hemisphere was brought some years 
ago under the notice of the St. Andrews Philosophical Society, and has since 
been variously discussed, and some good grounds urged for its acceptance. 
