ROCKS COMPOSED OP FOR AMINIFER A. 633 



racteristic of those organisms, but, when sections of them are 

 made sufficiently thin to be transparent, a spongy texture may 

 be most distinctly recognized in their interior.' It is curious 

 that many such sections contain well-preserved specimens of 

 Xanthidia, which are Desmidiacese whose divided body is covered 

 with long spinous projections, often cleft, and sometimes fur- 

 nished with hooks at their extremities ; and we occasionally also 

 find upon their surface, or even imbedded in their substance, 

 Foraminiferous shells (especially Rotalice), in which not only the 

 substanc'e of the shell has undergone silicification, but also that 

 of the soft animal body, the shrunken foiTxi of which may be re- 

 cognized in the dark carbonaceous hue imparted to the central 

 portion of the silex which fills each chamber. 



446. In examining Chalk or other similar mixed aggregations, 

 whose component particles are easily separable from each other, 

 it is desirable to separate, with as little trouble as possible, the 

 larger and more definitely organized bodies, from the minute 

 amorphous particles ; and the mode of doing this will depend 

 upon whether we are operating 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 levi- 

 gation already directed for separating the Diatomacese (§ 192). 

 It will usiially be found that the first deposits contain the larger 

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

 minifera and Sponge spicules fall next; the fine amorphous 

 particles 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 fioating 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 translu- 

 cent flint being first selected, and the chips that are obtained 

 being soaked for a short time in turpentine (which increases 

 their transparency), 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-struc- 

 ture, however, are only to be obtained by slicing and polishing, 

 — a process which is best performed by the lapidary. 



447. 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 



1 See Mr. Bowerbank's Memoirs in the " Transact, of the Geolog. Societ." 1840, and in 

 the " Ann. of Nat. Hist." 1st Ser. vols, vii, x. 



