324 GEOLOGY AND MINERALOGY. 



ower beds referred to it in Kansas might possibly be Jurassic ; and we are even now 

 prepared to believe it may yet be found to repose on Jurassic rocks in that Ter- 

 ritory, as it does at the Black Hills in Nebraska. 



FORMATION OF OOLITIC LIMESTONES BY ORGANIC AGENCIES. 



We translate the subjoined curious details respecting the formation of certain 

 recent limestones in Mexico, from a paper by M. Virlet d'Aoust, contained in a 

 late Bulletin of the Geological Society of France. After describing the -well 

 known position of the city of Mexico, and the lowering of the salt water lake of 

 Tezcuco by artificial drainage, the author proceeds as follows; — "The lowering of 

 this lake has necessarily exposed around its margin, the limestone deposit which 

 constitutes its bed. This limestone, slightly marly and of a greyish or pure white 

 color, is entirely of recent formation, as proved by the numei'ous fragments of 

 obsidian knife-blades {navajas de itztli) which occur in the neighbouring soil, and 

 which I have myself found imbedded in it, especially in the excavations at the 

 west of the city, made by M. Griffon the architect, for the foundations of a new 

 jail. I was struck by the oolitic texture which this limestone often presents, a 

 character not observed by me in the fresh water limestone deposit of Lake Ohalco. 

 The oolitic granules appear identical in aspect, form and size, with those of many 

 of the Jurassic oolites. I soon obtained an explanation of this structure. Being 

 one day with M. Guillemain at the house of a mutual friend, Mr. J. 0. Bowring, 

 the distinguished chemist and superintendent of the saltworks of Lake Tezcuco, 

 and calling attentien to the structure in question as seen in certain excavations 

 which were then being carried on, Mr. Bowring informed me that the oolitic 

 granules were nothing more than the eggs of a kind of fly, encrusted and cemented 

 together by the calcareous sediment of the lake, which is constantly under process 



of deposition This fact, the geological bearings of which were 



manifest, appeared to me so important, that I determined to verify it by personal 

 observation. I therefore returned to the lake in the month of October, the time 

 at which the eggs are chiefly deposited. I was accompanied by M. Guillemain, 

 and by the chemists M. Ernest Craveri and M. Poumar^de, who were no less 

 anxious than myself to witness this novel formation. "We were enabled to per- 

 ceive perfectly, in the shallower parts of the lake, the manner in which the eggs 

 were deposited. Myriads of little amphibious insects appeared upon the wing 

 in countless numbers, and plunging, from time to time, beneath the surface of the 

 water to the depth of several feet or even fathoms, they deposited their eggs 

 upon the bottom of the lake or on objects more conveniently within their reach ; 

 after which they withdrew themselves from the water and probably died. These 

 insects belong to the order Hemiptera, and constitute, according to M. Guerge 

 Meneville, two distinct species, belonging, indeed, to different genei-a. One, and 

 by far the more abundant of the two, is the Gorixa femorata. The other, the eggs 

 of which are larger, is the Notonecta unifasciata." M. Virlet d'Aoust remarks 

 also in the course of his memoir, that the encrustation of the eggs is facilitated by 

 the circumstance that each egg is attached to its sub-aqueous support, not directly, 

 but by the intervention of a short pedicel. The eggs, furthermore, are collected 

 by the natives in large quantities, and sold in cakes, as an article of food, under 



