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AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXII. — P re-Cambrian and Carboniferous Algal 

 Deposits; by W. H. Twesthofel. 



Not very long ago the view generally prevailed that, 

 except for their contributions to the coal series, plants 

 played "a very insignificant part in the building of sedi- 

 mentary rocks. The average textbook in geology still 

 gives the general impression that calcareous deposits 

 indicate accumulations of coral and shell and essentially 

 nothing is said relating to the contributions of the 

 plants. So strong was the view intrenched that calca- 

 reous deposits are of animal origin, that plant structures, 

 as Spongiostroma, Sphcerocodium, Cryptozoon and 

 others, were classed with the animals. 



The work of recent years is leading to the conception 

 that as rock-builders the plants — chiefly the algae — are of 

 prime importance. This work must ultimately compel 

 a restatement of the agents of limestone formation and a 

 general re-evaluation of the quantitative importance of 

 the several agents concerned. At one extreme, this work 

 relates to some of the oldest of rocks and, at the other, 

 to sediments now accumulating on the ocean floor. In 

 addition, considerable detailed work has been done in 

 intermediate portions of the geologic column, particularly 

 the Carboniferous of Great Britain and the Silurian and 

 Ordovician of the Scandinavian countries. 1 



"Walcott's explorations 2 of the Pre-Cambrian strata 

 together with contributions relating to the same rock 

 from other students have shown the great part that algae 



1 An excellent review of the algal deposits of the geological column is 

 given by Garwood, E. J.: Geol. Mag., N. S., Dec. V, vol. 6, pp. 440-446, 490- 

 498, 545-553, 1913. 



2 Walcott, C. D. : Pre-Cambrian Algal Flora, Smithsonian Misc. Coll., vol. 

 64, No. 2, 1914. 



Am. Jour. Sci.— Fourth Series, Vol. XL VIII, No. 287.— November, 1919. 

 24 



