50 



the more recent rocks resembling it is not diminished by the 

 fact that a portion at least of the ancient-looking petrosilex of 

 undoubted Huronian age contains 'petrosilex pebbles. These 

 pebbles are usually somewhat angular, often of the same color 

 as the enclosing rock or matrix, and although, as a rule, 

 sharply outlined, yet frequently very inconspicuous, requiring 

 weathering to make them visible. The inconspicuousness of 

 part of the pebbles seems to arise in some eases from their 

 identity with the enclosing rock, and sometimes from the partial 

 blending of the pebbles and paste, whereby the outlines of the 

 former are destroyed, and the pebbles, even when differing in 

 color from the paste, appear as mere ill-defined spots or blotches 

 in the latter. The pebbles seldom appear to be abundant or 

 evenly distributed, the rock rarely presenting the aspect of a 

 normal conglomerate or breccia. Concerning the true signifi- 

 cation of these pebbles three hypotheses naturally suggest 

 themselves : First, that they represent traces of a conglomerate 

 structure originally characterizing, perhaps, considerable por- 

 tions of the Huronian petrosilex. Second, that they have re- 

 sulted from the local disturbance and internal movement and 

 crushing of the petrosilex, and the subsequent partial cementa- 

 tion (lithologic regelation), under peculiar conditions of heat, 

 moisture, and pressure, of the fragments thus produced ; a sup- 

 position which, according to Prof. Pumpelly,^ accounts for the 

 breccia structure of portions of the petrosilex of Pilot Knob, 

 Missouri. Third, that the pebbles have become imbedded in 

 the petrosilex while the latter was in a liquid or plastic state, 

 perhaps undergoing extravasation. 



That the Huronian petrosilex is now for the most part 

 a stratified rock, and was originally wholly so, I cannot 

 doubt ; and it appears most probable that the conditions 

 presiding over its deposition differed in degree only, if at 

 all, from those that have obtained in more recent geologic 

 times. Extensive beds of Huronian conglomerate, it is well 



1 Geological Survey of Missouri: Preliminary Report on the Iron ores and Coal-fields, 

 1872. 



