72 
THE GEOLOGIST. 
iriuoral, is freshly takoa from the (jiiarry, it is often much softer or 
more fragile than when it has remained some time exposed to the air, as 
we have already remarked with regard to the emerald. This is very 
m;inifest, for instance, in newly-made marble slabs, which, if they are 
placed against a wall in an inclined position, are apt to bend, and become 
in a great measure valueless to the owners. After a few days' exposure 
to the air the rock or mineral becomes hard, losing at the same time a 
considerable quantity of water. M. Kuhlmann has endeavoured to 
prove that this hardening of the rock and expulsion of water are not 
owed to simple evaporation or drying, but to a process of crystallisation 
which takes place slowly from the moment the rock is exposed to the 
air. 
Natural crystals are often found strewed on a rock of their own com- 
position, which rock, in M. Kuhlmann's idea, has given birth to the 
crystals in question by a species of contraction or slow crj-stallisation, 
accompanied by loss of water — phenomena which he has artificially 
produced with sulphate of baryta, sulphuret of mercury, oxjde of 
cojjper, &c. He explains in like manner the origin of the beautiful 
crystals found in geodes. 
M. Virlet D'Aoust, civil engineer, has lately presented to the 
Academy of Sciences a paper on the eggs of certain aquatic insects 
found in Mexico, and which he looks upon as the means by which 
oolitic rocks have been, and are still being, formed. However extra- 
ordinary — we might, perhaps, say mysterious — this origin of oolites 
may appear, we must not be too hasty in rejecting the statements 
brought forward by the author, whom we believe to be a man of some 
geological experience, and a clever engineer. Has not Ehrenberg 
shown that immense masses of the earth's surface owe their origin to a 
profusion of microscopic infusoria, foraminiferae, &c. ? M. Virlet 
D'Aoust, in his turn, endeavours to show that oolitic rocks owe their 
existence to myriads of minute eggs, the seed of some aquatic insects. 
Here are the facts observed : — 
Everyone has heard of the great plain of Mexico, situated some 
2,300 meters above the level of the sea. Near the centre of this tract 
of land are seen two large lakes.* The first, the water of which is 
* It was from these large Mexican lakes that Humboldt brought back with him 
Scheuzher's antidiluvian man [homo diluvii testis), a large salamander belonging to 
the most recent freshwater formations. 
