308 THE MICROSCOPE AND ITS REVELATIONS. 



tissues which can be recognized in sections of nodular Flints, Agates," 

 etc., as to make it clear when taken in connection with correspondence 

 of external form that such flints are really fossilized sponges, the sili- 

 cifying material having been furnished by the solution of the skeletons of 

 the siliceous sponges, or of deposits of Diatoms or Radiolaria. Further, 

 in many sections of Flints there are found minute bodies termed Xan- 

 thidia, which bear a strong resemblance to the sporangia of certain Des- 

 midiacecB (Fig. 158, D); and the Author has found similar bodies in 

 the midst of what appears to be sponge-tissue imbedded in the Globige- 

 rina-mud. And (as was first pointed out by Mr. Sorby) the coccoliths 

 and coccospheres at present found on the sea-bottom ( 409), are often to 

 be discovered by the Microscopic examination of Chalk. 2 All these corre- 

 spondences show that the formation of Chalk took place under condi- 

 tions essentially similar to those under which the deposit of Globigerina- 

 mud is being formed over the Atlantic sea-bed at the present time. 



700. In examining Chalk or other similar mixed aggregation, whose 

 component particles are easily separable from each other, it is desirable 

 to separate, with as little ^trouble as possible, the larger and more defi- 

 nitely 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 lev- 

 igation already directed for separating the DiatomaceaB ( 300). It will 

 usually be found that the first deposits contain the larger Foraminifera, 

 fragments of Shell, etc., and that the smaller Foraminifera and Sponge-spi- 

 cules 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 011 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 trans- 

 parence), 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 performed by the lapi- 

 dary. 



701. 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; and the 

 presence of animals of this group may be largely recognized, by the 

 assistance of this instrument, in sections of Calcareous rocks of various 

 dates, whose other materials were fragments of Corals, Encrinite-stems, 



1 See Dr. Bowerbank's Memoirs in the "Trans, of the Geolog. Society,'' 1840 r 

 and in the "Ann. of Nat. Hist.," 1st Ser., Vols. vii., x. 



2 On the Organic origin of the so-called " Crystalloids " of Chalk; in " Amu 

 of Nat. Hist.," Ser. 3, Vol. viii. (1861), pp. 193-200. 



