84 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



danai,' which, hke the Atlantic coast lobster {Homarus americamis), 

 might have lived in quite cold water, or adapted itself to warmer, 

 muddy water, wdien cut off from marine waters. 



In China a cold period near the close of the Algonkian is sug- 

 gested by the presence of glacial deposits at or below the base of 

 the Man-t'o formation." 



ORIGIN OF ALGONKIAN LIMESTONES 



The origin of the great pre-Cambrian limestones of western Amer- 

 ica has long been a mooted question and the nature of the concre- 

 tionary-like Cryptosoon has not been so definitely determined as to be 

 accepted by common consent either as an alga or a Stromatoporoid. 

 Twenty years ago I had a number of thin sections made of the 

 matrix and " fossils " from the limestones of the Chuar terrane of 

 the Grand Canyon series of Arizona ' and later of specimens from the 

 Belt series of Montana. Not being able to discover any traces of 

 detailed or minute structure I put the specimens and slides aside for 

 future study. Recently I have had occasion to consider the question 

 of the origin of the magnesian Hmestones of the Algonkian forma- 

 tions of the Cordilleran area and in this connection to determine if 

 possible whether there was any relation between the so-called Cryp- 

 tozoons and the presence of the great series of limestones. 



As the thought that the entire Algonkian series of western 

 America were of epicontinental origin was forced upon me, I began 

 to doubt the marine origin of the limestones. It then occurred to 

 me to seek further information from the geologists who have been 

 studying the origin of fresh-water calcareous deposits and the 

 paleobotanists acquainted with the calcareous algae as active agents 

 in secreting and depositing the calcium and magnesian carbonate. 

 The result of these inquiries has led me to the conclusions that the 

 origin of the Cordilleran Algonkian limestones is largely owing to 

 the action of lime-secreting algae and bacteria and that precipitation 

 of calcium and magnesian bicarbonates from a saturated solution 

 is of very rare occurrence and not an important agent of deposition 

 in geologic time and that marine waters are not necessary for the 

 deposition of magnesian limestones. 



'Bull. Geol. Soc. America, Vol. 10. 1899, p. 239, pis. 25-27. 

 " Willis, Research in China, Vol. 2, 1907. p. 39. 



" Cryptozoan ? occidentale Dawson, Bull. Geol. Soc. America, Vol. 10, il 

 pp. 232-234, pi. 23, figs. 1-4. 



