142 ON THE COLLECTION AND 



the beauty of the siHceous shields it contains — Asteroniphalus 

 Brookei being especially deserving of notice. 



At Five Mile Canon, near Virginia City, Nevada, Dr. Meade 

 Edwards states that there is an enormously thick stratum of 

 Diatomaceje, which is ground in extensive mills, and sold in con- 

 siderable quantities as " Electro-silicon polishing powder." 



Ehrenberg, speaking of the vastness of some of these deposits, 

 draws our attention to one occurring on the banks of the Columbia 

 River, in North America : — " The Columbia, in its course at Place 

 da Camp, runs between two precipices composed of porcelain 

 clay, 500 feet thick, covered with a layer of basalt, on which some 

 volcanic stratum rests. The clay strata are of very fine grain, and 

 some are as white as chalk. Dr. Bailey has shown that this 

 apparently argillaceous layer is composed entirely of fresh-water 

 diatoms. Its perfect purity from sand proves that it is not a drift. 

 By its immense thickness of 500 feet, this layer of biolithic tripoli 

 far surpasses any similar layers elsewhere, which attain ordinarily 

 only one or two feet in thickness, although those of Luneberg and 

 BiHn have a depth of forty feet ; some beds we also know else- 

 where having seventy feet ; such are not pure, but are intersected 

 by layers of tufa, or other material." 



In turning nearer home we have some very pure fresh-water 

 diatomaceous deposits — that of Mull, in Scotland, being when dry 

 very soft and pulverulent; Premnay Peak, Glenshira sand, Lochs 

 Canmor and Kennard are, perhaps, the best of the series. 



In Wales, the ancient site of a mountain lake at Dogelly, and 

 Cwm Bycham furnish us with supplies of much the same character 

 as the foregoing. In Ireland, we have the well-known Mourne 

 Mountains, Bellahill, Stony Ford, Upper Bann^ Carrickfergus, 

 and Toome Bridge. These deposits are all well worthy of 

 notice. The prevailing forms here are Campylodiscus Hibernica^ 

 Surirella nobilis^ Cymatopleura Hibernica, Cocconema fiisiforma^ 

 Pin7iularia viridis^ Gomphonema constrida^ and one or two 

 forms of Melosira. 



The borings made by the late Mr. Okeden, at Neyland, a creek 

 near Milford Haven, Pembrokeshire, at a depth of thirty or forty 

 feet, revealed diatomaceous earths, very rich, in the remains both 

 of fresh -water and marine species; but I do not know of an 



