618 E. BLACKWELDER— ORIGIN OF BIGHORN DOLOMITE OF WYOMING 



tion, the structures we now find in the Bighorn dolomite. Even the little 

 winding tubes with which the Bighorn growths are perforated here and 

 there suggest the galleries of boring organisms which today rarely leave 

 reef structures of any kind undamaged. Also the variety of forms visible 

 in the Bighorn growths seems to parallel the wealth of species and facies 

 of the modern corallines as they have been observed in tropical seas. 



Since the positive identification of algae depends almost entirely on the 

 recognition of their delicate, cellular structures under the microscope, it 

 is unlikely that the problematical growths of the Bighorn terrane can ever 

 be satisfactorily recognized, inasmuch as nearly all traces of original or- 

 ganic structures have been obliterated. The cells of the modern algae, 

 such as Lithophylhim .-, average about .01" millimeter in diameter, but the 

 Bighorn rock is crystallized in grains .05 to .10 millimeter in diameter. 

 Under these circumstances we ought not to expect to find the algal cells 

 preserved. 



This disappearance of minute structures, one of the inevitable results of 

 the process of crystallization, may well explain the fact that marine algae, 

 although often reported from Paleozoic limestones, have in perhaps no 

 instance been satisfactorily identified from internal cell characters. As a 

 lowly and hence doubtless primitive group the antiquity of the algae may 

 be supposed to be great. We have therefore some a priori reason to be- 

 lieve that they were well differentiated and abundant in very remote geo- 

 logical periods, and also that the modern families are of relatively ancient 

 lineage. 



These facts have been discussed in some detail in order to show that the 

 modern coralline algae seem to fill the requirements of the case for the 

 Bighorn dolomite in that they are varied in form, grow in banks of great 

 extent and generally of singular uniformity, and that they are less subject 

 to limitations of depth and temperature than the corals. Possible objec- 

 tions on the ground of the absence of cellular structure or deficient pale- 

 ontological evidence that algae were abundant early in the Paleozoic era 

 seem to be met by satisfactory explanations. Therefore it appears to me 

 probable that the peculiar structures of the Bighorn dolomite are of 

 organic origin, and that the more massive coralline algae, such as the 

 modern genus Lithophyllum, may fairly be regarded as competent to 

 make such structures, if indeed they are not the only organisms which 

 could have done so. 



WET THE ROCK 18 A DOLOMITE 



The magnesian composition of such formations as the Bighorn dolomite 

 has apparently not yet been explained in such a manner as to be fully 



