ORIGIN OF THE STRUCTURE 615 



(given on page 612) show that the characteristic rough weathering dolo- 

 mite from several widely separated localities is a relatively pure rock con- 

 taining in no case more than 2% per cent of difficultly soluble matter, 

 such as silica, and in some instancees hardly a trace. Moreover, thin 

 sections when examined under the microscope would quickly show any 

 network of siliceous material such as Darton supposed to be the cause. 

 It seems to be true, however, that the pitted and ribbed surfaces are due 

 to differential weathering, although the latter has probably been induced 

 by differences in texture and porosity rather than in chemical composi- 

 tion. The very dense, fine-grained parts exclude dissolving solutions and 

 by their strong internal cohesion resist the abrasive action of the wind — 

 which in Wyoming is a major factor in rock wastage — while the more 

 porous, coarser-grained cement more readily admits carbonated waters 

 and its friable nature renders it more easily abradable. Etching tests 

 with hydrochloric acid on polished surfaces of typical specimens of the 

 rock show that the fine-grained parts are attacked much less rapidly than 

 the coarser parts. After several hours the process actually produced a 

 surface on which the dark dense bodies stood out with a relief of about 

 half a millimeter. We may now seek an explanation of the observed tex- 

 tural differences. 



In the absence of definitely recognizable organic structures, there may 

 be reasonable doubt as to whether the branching, ill-defined masses are 

 really of organic origin or not. The fact pointed out by Weed, that the 

 problematical bodies are darkened by nitrogenous matter which is largely 

 absent from the surrounding matrix, strongly suggests that the organic 

 material is residual and once formed a part of the bodies themselves. A 

 still more suggestive fact is the dichotomous habit of branching, which is 

 repeated ad infinitum among these structures in successive outcrops. This 

 and other matters of form are more or less significant of colonial organ- 

 isms, and particularly of certain types of corals and calcareous algae. It 

 is an unfortunate fact, however, that no trace of definite interna] structure 

 now remains. 



If the growths are in fact organic, what kind of organisms do they 

 represent? The great branching intergrown masses suggest thai Inquiry 

 be limited to the plantlike colonial types — among the animals the sponges, 

 corals, and bryozoans, and among the plants some of (lie calcareous algae. 

 If they were siliceous sponges, we might reasonably expect to find spicules 

 or a spicular network of chalcedonic quart/. The calcareous sponges have 

 the requisite chemical composition, but none that are known, either in the 

 modern or in the fossil state, seem to resemble in form the dark bodies in 

 the Bighorn dolomite, nor do they new build massive hanks comparable in 



