84 GEXEEAL HISTORY OF THE LNFUSOEIA. 



cubic metres of fine microscopic organisms, which, in the course of a century, 

 would give a deposit of from 720,000 to 1,400,000 cubic metres of infusory 

 rock or Tripoli stone." 



Another fact exemphfying the widely pervading presence of silicious In- 

 fusoria was revealed by the experiments of Ehrenberg, viz. their existence 

 in a h^ing state in moist earth beneath the surface, the only ^ital condition 

 necessary being a small quantity of moistiu^e. The presence of their remains 

 at considerable depths in mud also is well exemplified by the experimental 

 borings made by Mr. Okeden {J. M. S. 1854, p. 26) at Neyland, a creek of 

 IMilford Haven, where deposits rich in Diatomaceous remains of marine or 

 brackish and freshwater character occurred at the depth of 20, 30, and 

 40 feet. 



The preceding illustrations wiU suffice to show the active share taken by 

 the Diatomeae at the present day in the ever-occiu-ring changes of the earth's 

 siuface ; others must now be adduced to exemplify their influence in the past 

 physical changes of the globe. These examples are so numerous, and, relative 

 to other phenomena, so important, that it is embarrassing to make a selection. 

 Ehi-enberg is the most assiduous cultivator of this department of knowledge. 

 He has personally examined deposits collected from almost every countiy of 

 the world, and described, with illustrative plates, the genera and species he 

 has encountered in them, in his recent large work the Mihrogeologie, 1855. 

 One of the most striking and, to his mind, unique instances of a Diatoma- 

 ceous deposit, formed at a remote or geological period, he has shoAvn to exist 

 in North America, on the banks of the Colimibia Eiver. 



The river of Columbia, in its course at Place-du-Camp, rims between two 

 precipices 700 to 800 feet high, composed of porcelain- clay 500 feet thick, 

 covered over by a layer of compact basalt 100 feet thick, on which, again, some 

 volcanic deposits exist. The clay strata are of very fine grain, and vary in 

 coloiu' ; some are as white as chalk. Dr. Bailey has shown, from some por- 

 tions submitted to him by Col. Fremont, that this apparently argillaceous layer 

 is entirely composed of freshwater Infusoria. Its perfect purity from sand 

 shows that it is not a drift, but has been formed on the spot. By its immense 

 thickness of 500 feet, this layer of biohthic Tripoli far surpasses any similar 

 layers elsewhere, which attain ordinaiily only one or two feet thickness, 

 although those of Limebiu'g and Bilin have a depth of 40 feet. Some beds 

 we also know elsewhere ha\ing 70 feet ; yet such are not pure, but inter- 

 sected by strata of tufa or of other material. 



A very pm^e Diatomaceous deposit has been met with by Dr. Gregory in 

 the island of Mull, which when diy is almost white, and much resembles 

 chalk, being light, pliable, and adherent to the fingers {T. M. S. 1853, p. 93), 

 and in composition hardly contains anything besides silicious organic remains 

 '' for the most part entii^e, but with some fragments ; other portions which 

 are denser contain also many fi'agments of quartz of various sizes, and vast 

 numbers of comminuted fragments of loric?e." Prof. Smith {Sijnops. vol. i. 

 p. xii) says — "Districts recovered from the sea, in the present or other 

 periods of the earth's histoiy, frequently contain myriads of such exuviae 

 forming strata of considerable thickness." Examples of this nature in our 

 own coimtiy are met with in " the ancient site of a mountain lake in the 

 neighbourhood of Dolgelly, locahties of a similar kind near Lough Island- 

 Reavey in Down, and Lough Moume in Antrim." Mr. Okeden concludes, 

 from facts collected by borings in the mud of some creeks and rivers of 

 South Wales, " that not the smface merely, but the whole mass of these 

 tidal deposits is penetrated by these minute and wondrous organisms, while, 

 from the fact of their being foimd at Neyland at a depth of 40 feet below the 



