140 



DYNAiynCAL GEOLOGY. 



this mode of origin. The two sets of rings were either side of a crack iu 

 the rock, and had together a diameter of about twenty feet. 



Fig. 142 represents concentric areolets between mud cracks in an argilla- 

 ceous shale, made by siliceous waters at the time of the consolidation, when 

 the mud cracks were likewise filled with quartz, a layer of quartz being 



142. 



141. 



Concentric discoloration, Illewarra, N.S.W. D. '49. 



Concentric structure, Australia. D. '49. 



deposited against each wall. Whether in this case the concentric consolida- 

 tion was centrifugal or centripetal is not ascertained. Seashore wear of the 

 rock brought the structure to view. 



See further, on LitJiophysce, page 337. 



II. LIFE: ITS MECHANICAL WORK AND ROCK CONTRIBUTIONS. 



The making of rocks out of organic contributions, and the protective, 

 transporting, and destructive effects of life, are the subjects here under 

 consideration. 



General Remarks on Rock-making. 



1. Materials Aiforded by Plants and Animals. 



The organic contributions to rock-making are mentioned on page 71. It 

 appears that 



Plants afford — 



Calcareous material for rocks : mainly through Kullipores and Coccoliths, 

 and other calcareous Algae or the lowest of Cryptogams. 



Siliceous material : through Diatoms, and some conf ervoid Algae ; and spar- 

 ingly through other plants, the ashes of which afford some silica and alumina. 



Carbonaceous materials : through plants of all kinds, but especially those 

 that flourish in wet soils and marshes, where means of burial are convenient. 



Animals afford — 



Calcareous material : through Ehizopods among Protozoans ; Spongiozoans 

 with calcareous spicules, to a very small extent ; Actinozoans, or the Corals ; 

 Hydrozoans of the Hydroid section ; the lower Echinoderms or the Crinoids 

 and Cystoids, and other Echinoderms sparingly ; Molluscoids, as the Brachio- 



