Prof. Gregory on Diatomacese. in Soils. 221 



and rare species. It is very desirable that collectors of plants should 

 preserve a little of the earth adhering to their roots, end in this way 

 copious materials -would be obtained. 



The above observations entirely confirm Ehrenberg's statements as 

 to the distribution of the Diatomacece. They furnish endence of the 

 fact that these organisms are far less affected by climate and tempe- 

 rature than larger plants or animals ; since many of the very same 

 species are found in every latitude and in every country. For example, 

 such common forms as Achnanthidmm lanceolatuDt, Achnanthes 

 exilis, Gomphonema tenellum, G.consfrictnm, G. cajntatum, Cocconeis 

 Placentv.la, C. Pediculns, Cocconema lanceolatum, C. ctjmhifonne, 

 Synedra radknis, Navicula el/iptica, N. rhomboides, Pinnularia 

 viridis, P. major, P. oblonya, P. borealis, SurireUa biseriata, S. 

 ovata, Meridian circulare, M. constricium, Cymbella maculata, 

 C. scotica, C cuspidata, Epitliemia turyida, Ep. Aryus, Himan- 

 tidium Arcus, H. yracile, II. majus, Odontidium mesodon, Dia- 

 toma tenue, D. vidyare, Xifzschia linearis, iV. amp)hioxys, Me- 

 losira various, and many others actually occur in every part of the 

 world from whence these soils have come ; and there is absolutely no 

 difference between the exotic and the British forms. 



Ehrenberg specifies two species, namely Pinnularia borealis (P. 

 latestriata, W. G.) and Ennotia amphioxys {Nitzschia amphioxys, 

 W. Sm.), as having been found by him in almost every instance. My 

 results confirm this. In no one case have both of these been absent, 

 and in at least nine-tenths of these soils both are present. They are 

 often the predominant forms, and in a few cases almost the only 

 forms present. Gomphonema tenellum and Achnanthidium lanceo- 

 latum are found in a large majority of these soils. 



I am disposed to agree in opinion with Ehrenberg, that the micro- 

 scopic organisms found in soils contribute material!}- to the increase of 

 the soil. This is true both of the siliceous and calcareous forms. 

 The Diatomacece live in moist earth. They obtain silica froiu the 

 water, and at their death their shells are added to the soil. AVhere 

 many are present, this process of transference of silica from the rock 

 to the soil goes on very rapidly. We have so far evidence that they 

 live in these soils, that we find them there very often in the state 

 of self-division, which is not observed in old accumulations of the 

 dead shells. 



The peculiar capacity of the Diatomacece for resisting climatic 

 changes, whereby the same species can live and thrive as well in the 

 Arctic circle as under the line, corresponds well with the results of 

 the study of the same organisms in the fossil state. In Ehrenberg's 

 'Mikrogeologie ' will be found very fine figures of the Diatoms occur- 

 ring in the different forms of Bergmehl, Tripoli or polishing slate, 

 Kieselguhr, pumice, and other volcanic rocks, mountain limestone, 

 amber, &c., and it will be seen that by far the greater number of the 

 species are quite identical Avith recent ones. Microscopic organisms 

 have been found so low down as the green sand of the Silurian 

 system ; but they rather belong to the Pohjthalamia. The earliest 

 Diatoms, geologically speaking, as figured by Ehrenberg, agree in 



