CHALK, FLINT, AND CHERT 1089 



component particles are easily separable from each other, it is de- 

 sirable to separate, with as little trouble as possible, the larger and 

 more definitely organised bodies from the minute amorphous particles ; 

 -and the mode of doing this will depend upon whether we are operat- 

 ing upon the large or upon the small scale. If the former, a quantity 

 of soft chalk should be rubbed to powder with water by means of a 

 soft brush ; and this water should then be proceeded with according 

 to the method of levigation already directed for separating the 

 Diatomacece. It will usually be fojmd that the first deposits contain 

 the larger Foraminifera, fragments of shell, etc., and that the smaller 

 Foraminifera and sponge-spicules fall next, the fine amorphous par- 

 ticles 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 w r ater, with any particles still floating on it, should then be re- 

 moved ; 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 in- 

 -creases 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. 



The study of thin slices of flint and chert during late years 

 lias thrown much light on their origin and on the structure of 

 fossil sponges. Spicules are often found to be extremely abundant 

 as in the chert (Upper Greensand) from the quarry by Ventnor 

 station (Isle of Wight), where they can be detected by the naked eye. 

 The radiolaria from the Tertiary marl of Barbadoes have long been 

 known to microscopists, but these organisms more recently have been 

 detected in cherts. In Britain such cherts have been described 

 from the Ordovician rocks of Mullioii Island, Cornwall, and of south 

 Scotland, and the Carboniferous of south-west England. 1 



There are various other deposits, of less extent and importance 

 than the great chalk-formation, which are, like it, composed in great 

 part of microscopic organisms, chiefly minute Foraminifera ; 2 and the 

 presence of these may be largely recognised, by the assistance of the 

 microscope, in sections of calcareous rocks of various' dates, whose 

 other materials were fragments of corals, crinoid-stems, or the shells 

 of molluscs. In the formation of the Coralline Crag (Tertiary) of the 

 eastern coast of England, polyzoaries had the greatest share ; but 



1 On the former subject see G. J. Hinde, British Museum Catalogue of Fossil 

 Xponyes ; on the latter, the same, Quart. Journ. GeoL Soc. vols. xlvi. xlix. li. 



2 For illustrations of fossil foraminifera, see Carpenter, Introduction to Study of 

 Foraminifera (Ray Society), and the publications of the Palaeontograpliical 

 Society; Crag Foraminifera (T. Rupert Jones, &c.) ; Carboniferous and Permian 

 Foraminifera (H. B. Brady), The series also contains volumes upon the Crag 

 Polyzoa and various small Entomostraca of different ages. 



4A 



