191; Mr. H. C. Sorby on the Oi'ganic Origin 



form is peculiar to the chalk. In a note at p. 68^ he ascribes 

 them to the same kind of action as gave rise to the larger con- 

 cretions met with in limestone- and clay-deposits, and considered 

 the force which produced them not simple crystallization, though 

 in some respects analogous, and proposes for it the term ' Cry- 

 stalloid-Bildung.' The same idea is followed out in his paper 

 on Concretions, read at the Berlin Academy, June 29, 1840*, in 

 which he says he had endeavoured to make bodies like those in 

 chalk by artificial chemical means, but had not succeeded, though 

 he had made some to a certain extent similar. It must, hov/- 

 ever, be borne in mind that he looked upon them as fiat disks, 

 and not as curved in the manner shortly to be described. In his 

 magnificent work, 'Microgeologie' (Leipzig, 1854), he also figures 

 these ovoid bodies at pi. xxv. fig. b. 16, under the term ' Kreide- 

 Morpholithe,' along with various minute radiating groups of 

 crystals, evidently ascribing the whole to an inorganic action 

 more or less closely connected with crystallization. In order to 

 show to what extent such ovoid disks serve to make up some 

 varieties of chalk, he gives (at pi. xxx. b) a highly magnified 

 representation of the chalk of Rugen, and in various other 

 plates shows that they constitute a very large proportion of the 

 whole. It appears to me, however, a great exaggeration to afiirm 

 that chalk is composed of them, since a still larger part is made 

 up of particles which we may attribute with confidence to the 

 decomposed tissue of Foraminifera and other shells. 



The inorganic nature of the ovoid bodies of the chalk has 

 hitherto been almost universally adopted; for the only exception 

 I am acquainted with is the supposition of thellev.J. B.Readef, 

 who appears to have ascribed them to Infusoria. But when, about 

 ten years since, I commenced studying the microscopical struc- 

 ture of chalk, I soon became convinced that both these explana- 

 tions were unsatisfactory. By examining the fine granular 

 matter of loose, unconsolidated chalk in water, and causing the 

 ovoid bodies to turn round, I found that they are not flat disks, 

 as described and figured by Ehrenberg, but (as shown by the 

 oblique side view, fig. 5, p. 197) concave on one side and convex on 

 the other, and indeed of precisely such a form as would result from 

 cutting out oval watch-glasses from a moderately thick hollow 

 glass sphere whose diameter was a few times greater than their 

 own. This is a shape so entirely unlike anything due to cry- 

 stalline or any other force acting independently of organization — 

 so different to that of such round bodies, formed of minute ra- 

 diating crystals, as can be made artificially and do really occur 



* Neues Jahrbuch fur Mineralogie, &c. 1840, 680; Journal fiir prakt. 

 Chemie, 1840, xxi. 95; Ed. New Phil. Journ. 1841, xxx. 353. 

 t Mantell's Wonders of Geology, 2nd ed. vol. ii. 953. 



