338 PRINCIPLES OF STRATIGRAPHY 



aragonite spheres are converted into calcite, with often complete 

 retention of structure. The opinion of Rothpletz, who regards most 

 of these ooHtes as of organic (phytogenic) origin, will be given 

 later (Chapter XI). 



Deposits in Enclosed or Nearly Enclosed Basins. The 

 Black Sea (Andrussow-4) is characterized by relatively slight ver- 

 tical currents, owing to the superficial layer of water of less salin- 

 ity. This layer, 125 fathoms deep, is alone furnished with oxygen 

 from the surface, while below that the water is poisoned by the 

 gases resulting from the putrefaction of the dead animal bodies 

 which have sunk to the bottom from the surface layers, and which, 

 in the absence of scavengers, were left to decompose in the relatively 

 high bottom temperatures. Only anaerobic bacteria, especially Bac- 

 terium hydrosulfuricnin ponticiim, live in the lower strata of these 

 waters. At a depth of about 100 fathoms begins the separaffon of 

 H.,S by these bacteria, the quantity being 33 c.c. to 100 liters of 

 water, and rapidly increasing until at 500 fathoms it is 570 c.c. to 

 100 liters of water. Further down the increase is less rapid. Am- 

 monium carbonate is also produced in large quantities. The black 

 mud between 300 and 717 fathoms is charged with an abundant 

 separation of iron sulphide (FeS), according to the following reac- 

 tions (Murray and Irvine-36: pj, modified by Daly-ii : loj) : 



RSO, -f 2C = 2CO. + RS 

 RS + 2CO. +H0O = H,S + RCO3CO0 

 RS + RCd,CO, + H.,0 = 2RCO, + HoS 

 Fe^Oa + 3H2S = 2FeS + S + 3H,0 



With the iron sulphide and free sulphur there is also formed a 

 powdery precipitate of CaCOg, wdiich at a distance from shore, 

 where mechanical detritus is less abundant, forms layers and thin 

 banks of fine-grained limestone, whereas nearer the shore the 

 powdery lime is mixed with fine detritus. Shells of the planktonic 

 young of pelecypods and the frustules of pelagic diatoms are 

 likewise abundant in these deposits (Andrussow-4). 



Chemical Limestone Deposits of Lakes in Arid Regions. 

 While chemical precipitation of lime is found in the sea only under 

 exceptional conditions, and is of merely local significance, tl:e 

 chemical precipitation of lime from drying lakes in arid or semiarid 

 regions becomes a matter of greater import. The site of the former 

 Lake Lahontan in the Great Basin region (western Nevada) was 

 characterized by calcareous deposits of great extent, as was also 

 Lake Bonneville, the predecessor of Great Salt Lake. Three types 



