416 R. M. Field — Middle Ordovician of Central 



that the carbonate mud was probably formed by the 

 metabolism of the bacteria. I have examined this mate- 

 rial in the lagoons at Tortngas and find that it is not 

 quite so pure in every case but is apt to contain large 

 quantities of minute animal matter such as the tests of 

 Foraminif era, etc. It is very plastic, sun-cracks readily, 

 and when exposed to the air for a short time becomes 

 exceedingly hard, so hard in fact that it can not be easily 

 dissolved again in water. Its chemical and physical 

 characteristics are quite similar to those of the limestone 

 in the unfossiliferous zones of the Stones River group, 

 and I believe that a careful study of the shoal-water de- 

 posits off the southeast coast of Florida, especially in 

 the region of the Bahamas, will yield valuable data for 

 comparison with those of the Lower Paleozoics. 



"On the west side (of Andros Island) " according to Vaughan 

 ' ' there is an enormous flat, which is over 60 sea miles wide along 

 an east and west line, and on it the maximum recorded depth 

 is three and one-half fathoms. 



"It is probable that, especially during the summer months, 

 the temperature of the shoal waters is higher than on the surface 

 of the ocean where the depths are greater. Such an increase in 

 temperature would cause the water to lose C0 2 and produce 

 precipitation of CaC0 3 . Surface agitation of the water would 

 accelerate the loss of C0 2 and thereby increase the rate of pre- 

 cipitation of CaC0 3 ." 



"From the foregoing discussion it is obvious that there are 

 at least three cooperating factors tending to produce precipita- 

 tion of CaC0 3 , viz: (1) Ammonifying bacteria, (2) concentra- 

 tion of salts in solution through evaporation, (3) expulsion of 

 C0 2 by increase in temperatures. As these factors have not 

 been evaluated, a satisfactory solution of the complicated prob- 

 lem awaits further research" (op. cit., pp. 273-274). 



Although the Valentine limestone has not the charac- 

 teristic "Birdseye" appearance, the thin, thread-like 

 particles of pure calcite which are clearly seen in the 

 hand specimen may be evidence of the former existence 

 of a marine plant, or Nullipore, which flourished at the 

 time of the deposition of the fine, limy muds. The lack 

 of any definite organic structures, however, either in 

 the hand specimen or under the microscope, makes this 

 hypothesis even more indefinite than that regarding the 

 Lowville of New York. It is therefore suggested that 

 the original oozes out of which the Valentine limestone 

 resulted may have been formed, partly at least, by the 

 metabolism of some low form of organism similar to 



