232 PHYSIOGRAPHY. [chap. 



hard envelopes contributes largely to shallowing the water 

 and blocking up harbours. It was estimated by Ehrenberg 

 that as much as 18,000 cubic feet of siliceous organisms were 

 accumulated every year in the harbour of VVismar, in the 

 Baltic. Sir J. Hooker refers to the enormous multitudes 

 of diatoms in the waters and in the ice of the South Polar 

 Ocean ; and a deposit, or ooze, consisting principally of their 

 siliceous casings, was found along the flanks of the Victoria 

 Barrier, extending over an area which measured as much as 

 400 miles in length and 200 in breadth. During the voyage 

 of the Challenger, a similar diatomaceous ooze was found, 

 as a pale straw-coloured deposit, in certain parts of the 

 Southern Ocean ; and diatoms are seen in abundance on 

 the surface of many seas, especially where fresh water is 

 brought down by rivers. Insignificant as diatoms appear 

 when regarded individually, it is clear that, by their vast num- 

 bers and by the comparative indestructibility of their cases, 

 they may play a very important part in the formation of certain 

 deposits which will eventually constitute siliceous rocks. In 

 fact, Ehrenberg has shown that the loose siliceous particles 

 of the diatomaceous deposit at Bilin may become altered 

 into a compact rock by the percolation of water. This very 

 slowly dissolves the silica and then re-deposits it, as a hard 

 opal-like rock, in which the organic structure is well-nigh 

 obliterated. 



There are not many plants that have power, like the 

 simple diatoms, to encase their cells in such hard material 

 as silica. In the Grasses, however, the cells forming the 

 coating of the stalks contain a good deal of silica, which 

 confers rigidity upon the structure ; and one of the Horse- 

 tails ^ IS so rich in this substance that it is imported from 

 Holland, under the name of " Dutch rushes," for use as a 

 1 Eqiiisetum hycmale. 



