HANDBOOK OF SEA-WEEDS. 25 



such have been found as low down as the Silurian formation, 

 although their identity has been disputed, and several of them, 

 it is more than probable, belong to other orders, and some even 

 to the animal kingdom. Freshwater forms, all of existing genera 

 and species, are believed to have been detected in the carboni- 

 ferous rocks of Britain and France ; others also of the green- 

 coloured division are said to occur from the Silurian to the 

 Eocene, and the Floridese to be represented from the Lias to the 

 Miocene. The indestructible nature of the shells of the Diato- 

 maceae has enabled them to survive where the less protected 

 species may have perished. Tripoli stone, a Tertiary rock, is 

 entirely composed of the remains of microscopic plants of this 

 tribe. It is from their silicious shells that mineral acquires its use 

 in the arts, as powder for polishing stones and metals. Ehren- 

 berg estimates that in every cubic inch of the tripoli of Bilin, in 

 Bohemia, there are 41,000,000 of Gaillonella distans. Districts 

 recovered from the sea frequently contain myriads of Diatoms, 

 forming strata of considerable thickness ; and similar deposits 

 occur in the ancient sites of lakes in this and other countries. 



Before setting out in search of Algae the collector ought to 

 provide himself with a pair of stout boots to guard his feet from 

 the sharp-pointed rocks, as well as a staff or pole to balance 

 himself in rock-climbing, which ought to have a hook for draw- 

 ing floating weed ashore. A stout table-knife tied to the other 

 end will be found very useful. A basket a fishing-basket does 

 very well or a waterproof bag, for stowing away his plants, is 

 also necessary. It is advisable to carry a few bottles for the 

 very small and delicate plants, and care should be taken to keep 

 apart, and in sea-water, any specimens of the Sporochnaceae ; for 

 they are not only apt to decay themselves but to become a cause 

 of corruption in the other weeds with which they come in contact. 

 These bottles should always be carried in the bag or pocket, 

 never in the hand. 



Sea-weeds, as every visitor to the coast knows, are torn up 

 in great numbers by the waves, especially during storms, and 

 afterwards left on the shore by the retiring tide. Many shallow- 

 growing species are also to be found attached to the rocks, 

 and in the rock pools, between high and low water mark. 

 There are three points on the beach where the greatest ac- 

 cumulations of floating Algae are found : high water mark, mid 

 tide level, and low water mark. Low water occurs about five or 

 five and a half hours after high water. The best time for the 

 collector to commence is half an hour or so before dead low 



