72 THE GEOLOGIST. 



nnneral, is freshly taken from the quarry, 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 

 manifest, 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 crystallisation, 

 accompanied by loss of water — -phenomena which he has artificially 

 produced with sulphate of baryta, sulphuret of mercury, oxyde of 

 copper, &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. Yirlet 

 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 

 Si-houzher's anddilurian man (homo diluvii testis), a large salamander belonging to 

 the most recent freshwater formations. 



