204 



THE GEOLOGIST. 



in the Hartz, where, it is well known, many minerals are found in which selenium 

 and some other elements are combined, sU*ch 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 of Flint-Pebbles, — Dear 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 resemblanee 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 pu,t together by replacing the fragments ; 

 and if held firmly in the hand will exhibit scarcely any traces of the numerous 

 cracks. — Yours truly, Edmund Jones. 



Crenate op Ammonia in an Oligist oe 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 insects, 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 tais 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 oe 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 l lie subject of former climatal changes. — E. Weld, Doncaster. 



This matter has jusd 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 

 m armor 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. 



