THE GEOLOGIST. 



73 



fresh, goes by the name of Chalco, the second is a saltwater lake called 

 Texcoco. The observations made by M. Yirlet led him to discover that 

 the bottoms of both these lakes are formed by a sort of grey limestone 

 of modern formation, containing small oolites which, in the author's 

 eyes, are in every respect similar to those found in the rocks of the 

 Jura formation. He immediately made known this fact to Mr. 

 Bo wring, director of the salt works at Texcoco, who informed him that 

 these oolites were owed simply to the incrustation of the eggs of water 

 insects by the carbonate of lime daily deposited from the waters of the 

 lakes. In a second excursion to the lakes the author remarked that 

 their banks were strewed, under water, with an infinite number of 

 insects eggs, about the size of a pin's head, and which appeared to be 

 those of a new species of boat-fly (JSTotonecta). The Mexicans consume 

 immense quantities of these eggs at their meals. They are extracted 

 from the water by means of bundles of reeds or grass, on which the 

 insects deposit their eggs by millions at a time. M. Yirlet is not only 

 convinced that these modern oolites of Mexico owe their origin to the 

 eggs of a species of boat-fly, but seems to think also that the oolite 

 of the Jura, and other ancient strata, must be attributed to a similar 

 cause. This explains," says he, the irregular distribution of oolitic 

 grains in the rocks of the Jurassic strata. Where the oolite is hollow, 

 the egg has been enclosed before being hatched ; where the oolitic 

 globules are completely solid, the eggs have had time to hatch, and the 

 cavities left by the exit of the larvce have been filled up by the in- 

 crusting limestone. 



If these facts are confirmed by future observation, it will not be 

 without interest that we shall recall the Greek origin of the word 

 oolite (O ov, egg 5 \lOos, stone). I would, however, on this occasion 

 remind our readers that a small oolitic bed, bearing great resemblance 

 to the Jura limestone, was formerly discovered by Leopold von Buch 

 near Teguiza in Lancerote — one of the Canary Islands. This oolita 

 bed is also of modern formation, and probably continues increasing at 

 the present time. It would, therefore, be of great geological interest 

 to ascertain if the oolitic deposit made known to us by Leopold von 

 Buch owes its origin to causes similar to those stated by M. Yirlet in 

 reference to the Mexican oolite. Such an investigation, which could 



Canarislie inseln. 



I 



