PROTOZOA AS ROCK-BUILDERS. 81 



in the south ; while the intermediate zone would be covered 

 with Globigerina ooze, containing a comparatively small pro^ 

 portion of silicious matter. The thickness of the calcareo- 

 silicious and silicious beds thus formed would be limited only 

 by time and the depth of the ocean. These strata, once ac- 

 cumulated, would be liable to all those influences of percolat- 

 ing moisture and subterranean heat which are known to suf- 

 fice to convert silicious matters into opal, or quartzite, and 

 calcareous matters into the various forms of limestone and 

 marble. And such metamorphic agencies might more or less 

 completely obliterate the traces of their primitive structure. 



But yet other changes might be effected. At the present 

 day, in the Gulf of Mexico, off the Agulhas Bank and else- 

 where, at no great depths (100 to 300 fathoms) the Fora- 

 miniferal mud is undergoing a metamorphosis of another 

 character. The chambers of the Foraminifera become filled 

 by a green silicate of iron and alumina, which penetrates into 

 even their finest tubuli, and takes exquisite and almost in- 

 destructible casts of their interior. The calcareous matter is 

 then dissolved away, and the casts are left, constituting a 

 fine dark sand, which, when crushed, leaves a greenish mark, 

 and is known as '' green-sand." 



Moreover, the researches of the Challenger have shown 

 that in great areas of the Atlantic and Pacific Oceans over 

 which the sea has a depth exceeding 2,400 fathoms — areas in 

 some cases of many thousand square miles in superficies — the 

 bottom is covered not by Globigerina ooze, but by a fine red 

 clay, which is also a silicate of iron and alumina. In this clay 

 no remains of Glohlgerhia or other calcareous organisms are 

 found ; but, where these great depths gradually pass into shal- 

 lower water, they make their appearance in a fragmentary 

 condition — gradually becoming more and more perfect as the 

 depth diminishes to 2,400 fathoms or thereabouts. 



Nevertheless the Glohlgerinm and other Foraminifera 

 abound at the surface over these areas as they do elsewhere, 

 and their remains must be rained down upon it. Why they 

 disappear, and what relation the red-clay mud has to them, is 

 a problem not yet satisfactorily solved. It has been suggested 

 that they are dissolved and that the red clay is merely the 

 insoluble residue, left after the calcareous portion of their 

 skeletons has disappeared. In this case the red clay, like the 

 Globigerina ooze, the silicious mud, and the green-sand, will 

 be an indirect product of living action. 



Metamorphic processes operating upon clay, how^ever, may 



