THE GEOLOGIST. 
73 
fresh, goes by the name of Chalco, the second is a saltwater lake called 
Texcoco. The observations made by M. Virlet 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 (Notonecta). 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. Virlet 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 tliink 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 larvae 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 (12ov, egg ; Xi^os, 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 oolite 
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. Virlet in 
reference to the Mexican oolite. Such an investigation, which could 
* Canarishe inseln. 
