io68 PRINCIPLES OF STRATIGRAPHY 



Clinton iron ore also contains a somewhat dwarfed fauna which 

 may have been due to the presence of much iron in the sea water. 



Dwarfing effects due to an abundance of H.S in the water are 

 shown in the Black Sea, where, owing to the slight vertical cur- 

 rents, much stagnation occurs, and much HoS is separated out 

 (see ante, Chapter IV). Here life below a certain depth is prac- 

 tically absent, while the bottom deposits are strewn with young 

 shells from the plankton. The dwarf faunas of the Palaeozoic 

 Black shales have been considered as produced by conditions such 

 as now exist in the Black Sea, but the depth during the formation 

 of most of them was probably slight, though stagnation was no 

 doubt marked. 



The presence of mud or other mechanical impurities likewise 

 exerts a dwarfing effect on many organisms. This is illustrated 

 by the fauna of the eastern part of the Mediterranean, where a 

 large quantity of fine mud brought m by the Nile is held in suspen- 

 sion. (De Lapparent-i2:7ji'.) Shimer thinks that the dwarfed 

 faunas of the late Siluric rocks of eastern New York (Rosendale, 

 Cobleskill, Rondout, Manlius) are due to the presence of an abun- 

 dance of lime-mud, of the same kind of which the strata are com- 

 posed. This is a general characteristic of Palaeozoic calcilutytes. 



Dwarfing of organisms due to a floating habitat is seen in the 

 case of a California coast Pecten (F. latianritus), which when 

 growing near the coast is large and more strongly sculptured than 

 when fastened to floating kelp far from the coast (var. fucicolus). 

 The molluscs, living on and among the sea-weed which crowds the 

 eastern shallower part of the harbor of Messina, are throughout of 

 smaller forms, but are present in enormous number of individuals. 

 (Fuchs-15 :i'o./.) Walther (56) remarks upon this that the physi- 

 cal conditions of a special type of plant life here cause indirectly 

 the origin of the micro-fauna. The micro-fauna here is not so 

 much a dwarf fauna as one due to selection of small species by the 

 peculiar characteristics of the habitat. 



The influence of variation in temperature on the size of tiie 

 individuals is illustrated by the experiment of Semper (44) on 

 Lunncva stagualis, in which he found that growth began at a 

 temperature of 12° C, but that a lower temperature retarded or 

 completely arrested growth, though not affecting the life of the 

 animal. 'Tf," says Semper "a Limna^a came to be placed in a 

 pool or stream where for only two months of the year the tempera- 

 ture is higher than the minimum (12° C), growth will be checked 

 throughout the greater part of the year, and a diminutive race 

 result, since sexual maturity cannot be reached with a lower tern- 



