128 INTRODUCTION TO CEYPTOGAMIC BOTANY. 



constituting a bank which stretches two hundred miles north 

 from the base of the Victoria barrier, while the average depth 

 of water above it is three hundred fathoms, or one thousand 

 eight hundred feet."* 



100. But not only are species found in a fossU state which are 

 identical with recent forms, but the trade winds bring over 

 large quantities in what Ehrenberg calls Passatstauh, or 

 trade wind-dust, which, sinking through the upper current, 

 streaming from America, into the lower stratum, falls in the 

 form of fine powder. This, however, does not contain for the 

 most part African forms, though apparently coming from that 

 coast, but truly American species, many of which are identical 

 with those from the Antarctic. Another curious fact is, that 

 they are thrown up with volcanic ashes from active craters; 

 but the same power which in certain instances would carry the 

 fish from the sea, would be sufficient to waft such far less 

 perishable bodies, in a state capable of recognition. It would 

 take many pages to go through the different lists of species 

 peculiar to the different parts of the globe ; many forms, how- 

 ever, which do not occur at present in our own country, are 

 abundant in certain fossU deposits, and in exotic lands, but by 

 no means to the exclusion of Mediterranean forms from the cold 

 southern latitudes. In the collection made by Dr. Hooker in 

 the Himalayas, whether in hot springs, or in the mountain frosts 

 and streams, the species resemble closely our own ; and probably, 

 when they shall have been sufficiently examined, few new species, 

 and certainly no novel forms, will be detected. A few instances 

 will shew in what distant localities the species are found. E'pitlie- 

 Tnia gibba, a very common British species, occurs also in Italy 

 and Ceylon ; Epithemia Westermanni at Warehamand Ceylon ; 

 Actinocyclus undulatus, Kiitz., in various places in England, 

 also in Peruvian guano, and in Virginia; Triceratum alternans, 

 BaU., at Poole and Folkestone, and also in Peruvian guano. 



101. It is quite impossible to notice every form, we must 

 therefore content ourselves with passing in review merely such 

 as may be most characteristic. There are three principal groups, 



* There is a bed of Diatomacece, at Eiolimond, in Virginia, on whicli 

 the town is built, eighteen feet tliick. 



