ORGANIC CONSTITUENTS OF ROCKS. 759 



If the former, a quantity of soft Chalk should he ruhhed to 

 powder with water, by means of a soft brush ; and this water 

 should then be proceeded-with according to the method of leviga- 

 tion already directed for separating the Diatomaceae (§ 237). It 

 will usually be found that the first deposits contain the larger 

 Foraminifera, fragments of shell, &c, and that the smaller Fora- 

 minifera and Sponge -spicules fall next ; the fine amorphous parti- 

 cles remaining diffused through the water after it has been standing 

 for some time, so that they may be poured-away. The organisms 

 thus separated should be dried and mounted in Canada balsam. — 

 If the smaller scale of preparation be preferred, as much Chalk 

 scraped fine as will lie on the point of a knife is to be laid on a 

 drop of water on the glass slide, and allowed to remain there for 

 a few seconds ; the water, with any particles still floating on it, 

 should then be removed ; and the sediment left on the glass should 

 be dried and mounted in Balsam. — For examining the structure 

 of Flints, such chips as may be obtained with a hammer will 

 commonly serve very well : a clear translucent flint being first 

 selected, and the chips that are obtained being soaked for a short 

 time in Turpentine (which increases their transparence), those 

 which show organic structure, whether Sponge-tissue or Xanthidia, 

 are to be selected and mounted in Canada balsam. The most 

 perfect specimens of Sponge-structure, however, are only to be 

 obtained by slicing and polishing, — a process which is best per- 

 formed by the Lapidary. 



591. There are various other deposits, of less extent and impor- 

 tance than the great Chalk -formation, which are, like it, composed 

 in great part of Microscopic organisms, chiefly minute Forami- 

 nifera ; and the presence of animals of this group may be recog- 

 nized, by the assistance of this instrument, in sections of Calca- 

 reous rocks of various dates, whose chief materials seem to have 

 been derived from Corals, Encrinite-stems, or Molluscous shells. 

 Thus in the 'Crag' formation (Tertiary) of the eastern coast of 

 England, the greater portion of which is perceived by the unas- 

 sisted eye to be composed of fragments of Shells, Corals (or rather 

 Polyzoaries, § 445), and Echinodermata, the Microscope enables us 

 to discover Foraminifera, minute fragments of Shells and Corals, 

 and spicules of Sponges ; the aggregate being such as is at present 

 in process of formation on many parts of our shores, and having 

 been, therefore, in all probability, a ' littoral ' formation ; whilst the 

 Chalk (with other formations chiefly consisting of Foraminifera) 

 was deposited at the bottom of deeper waters. Many parts of the 

 Oolitic (Secondary) formation have an almost identical character, 

 save that the forms of Organic life give evidence of a different age ; 

 and in those portions which exhibit the ' roe-stone ' arrangement 

 from which the rock derives its name (such as is beautifully dis- 

 played in many specimens of Bath-stone and Portland-stone), it is 

 found by Microscopic examination of transparent sections, that 



