CHAPTER XL 



CHARACTER AND LITHOGENESIS OF ORGANIC OR BIOGENIC 

 ROCKS— (CONTINUED). PHYTOGENIC DEPOSITS. 



Phytoliths, or rocks formed from the remains of plants, may 

 be classed with reference to their material as acausto phytoliths and 

 caustophytoliths. The latter comprise the peats, coals, bitumens, 

 etc., while the former include calcareous or siliceous mineral matter 

 secreted by the growing organism. With reference to their source 

 of origin, phytoliths may be classed as autochthonous, or such as 

 arc deposited where they grew, or allochthonous, those brought 

 from other localities and so deposited in foreign soil. 



Autochthonous acaustophytoliths have been mentioned in the 

 preceding chapter, in the growth of nullipores on coral reefs, and 

 allochthonous acaustophytoliths have been described in the diatoma- 

 ceous oozes. In this chapter we shall discuss more fully the two 

 classes of phytoliths, beginning with those depositing mineral mat- 

 ter other than carbon. 



ACAUSTOPHYTOLITHS. 



Deposits Formed by Lime-secreting Alg^. Among the plants 

 belonging to the comparatively low division of alg?e are a number 

 which secrete carbonate of lime or silica and deposit it either as a 

 coating on the exterior or within their tissues. 



Modern Marine Forms. Among the lime- and silica-secret- 

 ing algse of the modern ocean the following may be especially men- 

 tioned. 



Order Cyanophycece, or Blue-Green Algce. 



This order contains several lime-secreting members, some of 

 which become important as rock builders. 



Phytogcnic Oolites. On the shores of Great Salt Lake, Utah, 



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