42 COLLECTING DIATOMS. 



S.W.iW. or S.S.W.|W. direction, and which is in part dependent on the sea 

 wave coming from the north, and in part on waters of the English Channel 

 returning again. (') 



The pelagic gathering mentioned above, made during the flowing tide, 

 also shows its southern origin by the diatoms of which it is composed. 



C. Collecting from Fossil Deposits. 



Fossil deposits may be divided into marine and fluvial deposits. 



Marine deposits. " Gather all earths," writes our friend Dr. Arthur M. 

 Edwards, "of light colour, varying from a pure white, through different 

 shades of grey, cream, and fawn to an iron-rust tint. The texture is often 

 friable, and then looks somewhat like clay, especially when it is wet ; at 

 other times it is of a hard and stony character, although always more or 

 less porous, and when soft, of little weight. Collect enough to make up 

 three or four pounds weight, or, say, a block six or eight inches square, 

 and, if possible, at various depths, because often these deposits vary in 

 character according to the depths at which the gatherings are made. Every- 

 thing that can be ascertained with regard to their position and their 

 relation to other beds should be noted. Also any fossils contained in 

 them or in the strata above or below them ; if they are not known they 

 should be taken in order to fix their species later. All specimens should 

 be kept carefully separate (not even permitting them to come in contact) 

 by wrapping each one in paper, placing within a label having written upon 

 it in ink the exact locality, date of collection, and name of collector. 

 It is also desirable that note should be made of the depth from the 

 surface at which the specimen was taken, together with any other infor- 

 mation that may be deemed of interest, as supposed extent of stratum, 

 slope-upwards towards north, south, east, or west, and thickness." 



Guanos. In conjunction with marine diatoms we ought here to mention 

 guanos which (as is well known) consist of the excrements of web- 

 footed birds found in warm regions of the globe, especially Peru, the 

 Chincha Island, Bolivia, &c. The birds which produce guano feed on 

 fish and various marine products, all of which contain diatoms or have 

 them on their surface, and consequently diatoms may often be found in these 

 guanos in large quantities ; by properly treating such guanos, many rare 

 forms can be obtained, which would elsewhere be sought for in vain. 

 Unfortunately, the best guano deposits are rare, or becoming so, and 

 commercial guanos are so often adulterated. 



(') Petit, op. cit., p. 54. 



