NO. 2 PRE-CAMBRIAN ALGONKIAX ALGAL FLORA 97 



The degree of replacement is variable, and ranges from less than one per 

 cent to 45.65 per cent, although most commonly found to be either low or 

 high. Limestone containing a higher percentage of magnesium carbonate 

 than true dolomite may be termed " super-magnesian " limestones, and if all 

 the calcium carbonate is replaced by magnesium carbonate the rock becomes 

 magnesite. This process of replacement is known as dolomitization, and is 

 accompanied by contraction or shrinkage of alDOut 12.3 per cent' of the volume 

 of the original limestone. This contraction is believed to produce porosity in 

 the rock under conditions where the pressure is not sufficiently great to close 

 the pores of the rock. 



THE BIOTIC RECORD 



The fmma.— The biotic record and character of the Algonkian 

 rocks included in the Grand Canyon, Llano, and Belt series of the 

 Cordilleran region, and in formations correlated with them,^ prove 

 that the marine waters of the extra-continental seas very rarely had 

 access to the epicontinental seas and lakes in Algonkian time. Such 

 connection appears to have been established in mid-Beltian time 

 when at least a crustacean, and a few annelids penetrated into and 

 became adapted to the conditions of the Montana-Alberta sea. and 

 more or less similar forms to the Arizona sea.* Other and different 

 forms may have lived in these and other interior bodies of water, 

 but as yet we have no knowledge of them. 



The vertical range of the small Beltian (Algonkian) fauna is 

 limited to a few hundred feet of strata in the Cordilleran area, a 

 fact which tends to demonstrate that the environment was not favor- 

 able to its development and survival for any considerable period. 



The most satisfactory explanation of the absence of a character- 

 istic marine life in Algonkian deposits is the probability that all the 

 known rocks of Algonkian time are of non-marine origin and hence 

 could not have had the opportunity to embed a marine fauna except 

 as few marine species gained access to the epicontinental seas and 

 quickly disappeared. 



The existence of a large and varied marine life (Lipalian) in the 

 extra-continental pre-Cambrian seas is inferred from the occurrence 

 of a highly organized and varied fauna in Lower Cambrian time in 

 both the Cordilleran and Appalachian geosynclines. The worldwide 

 distribution of the Lower Cambrian fauna also indicates the great 

 antiquity of the fauna from which it was derived. 



^Van Hise, C. R., A treatise on metamorphism; U. S. Geol. Survey, 

 Monogr. 47, 1904, p. 806. 

 ' Bull. U. S. Geol. Survey, No. 350, 1909, PP. 42-46. 

 ' Bull. Geol. Soc. America, Vol. 10, 1899, pp. 199-244- 



