616 E. BLACKWELDER — ORIGIN OF BIGHORN DOLOMITE OF WYOMING 



their vast extent to those of the problematical organism of the Bighorn. 

 It is difficult to believe that the obscure growths are coralline, inasmuch 

 as the rock contains many corals and in all of them the characteristic 

 structures are still distinct, although now seriously blurred by recrystalli- 

 zation. These more or less well-preserved corals have been found im- 

 bedded in the cement among the problematical growths throughout the 

 Bighorn formation. The appearance and state of preservation of the 

 coral structures is notably unlike that of the dark problematical bodies 

 which are the subject of this inquiry. The bryozoans also seem unsuited 

 to fit the demands of the case, partly because in most rocks they are pre- 

 served with more or less well-defined structural details and partly also 

 because they are usually small, delicate, and measurably independent, 

 whereas the Bighorn growths are thick, coarse bodies and evidently ex- 

 isted in enormous masses. On the whole no group of animals seems fitted 

 to serve the needs of the case in a satisfactory way, although they are not 

 positively excluded by the available evidence. 



An examination of the calcareous algae from the same viewpoint leads 

 to more favorable results. Although the majority of marine algae contain 

 no hard parts whatever, there are many which are either incrusted with 

 lime carbonate or secrete solid stony masses comparable to those made by 

 the reef-building corals. Among the modern calcareous algae a few con- 

 tain lime carbonate in the form of aragonite, whereas in the others the 

 mineral is calcite. Although these two minerals are apparently identical 

 in chemical composition, they have sufficiently different physical proper- 

 ties that it becomes important to discriminate between them. Thus ara- 

 gonite is more soluble than calcite, is more readily altered, has a higher 

 specific gravity, poorer cleavage, and a different electrical potential. 



According to Engler and Prantl, 13 the calcareous algae belong princi- 

 pally to three great classes: the blue-green algae (Chlorophycece) , the 

 Stoneworts (Characece), and the red algae (Rhodophycece) . 



Among the blue-green algae several genera, of the Dasycladacece and 

 Codiacea have more or less calcified or incrusted thalli. The typical 

 genus C odium assumes a cylindrical stalklike form with many branches, 

 but is not calcified. In the Bighorn dolomite there are branching growths 

 which bear an external resemblance to C odium, although there is no posi- 

 tive evidence that the two are related. Halimeda, one of the best known 

 and most abundant of the calcified blue-green algae in modern seas, con- 

 of loosely articulated fan-shaped segments incrusted with aragonite. 

 Xo forms observed in the Bighorn dolomite are directly suggestive of this 

 genus. Indeed it is not clear that after the decomposition of the proto- 



"Dic Natlirlichen Pflanzenfamllien, Tell 1, Abteilung 2, 1897. 



