70 



no more matter than a small grain of sand ; and, even suppos- 

 ing that the larger layers may have resulted from the flattening 

 of pebbles, it seems contrary to reason that the deformation 

 should have extended to the finer materials constituting the 

 paste, reducing and extending each minute grain in the same 

 proportion as the larger masses. A degree of plasticity which 

 would permit the flattening of the large pebbles would appar- 

 ently tend to equalize the pressure on different sides of the 

 smaller particles. And probably none of the material, coarse 

 or fine, could experience flattening save where there was room 

 for the lateral expansion of the rock ; where the rock was con- 

 fined on all sides, mutual indentation of the pebbles might fol- 

 low the application of enormous pressure, but there could be 

 no sensible flattening. The " flattened pebble" petrosilex, if 

 derived from the breccia, must have had its lateral extension 

 increased at least two-fold. Such a prodigious squeezing out 

 may be conceivable in a limited mass like the rock in question ; 

 but it certainly passes the bounds of probability where the mass 

 affected is miles in extent, as is the banded petrosilex in New- 

 bury. It will probably be claimed, however, that what I am 

 here unwilling to admit for the petrosilex is a well-known and 

 demonstrable fact for many cleaved slates, in which fossils and 

 even pebbles have suffered great distortion, although the cleav- 

 age is mainly due to the flattening in parallel directions of the 

 minute granules of the rock ; but this objection appears of 

 little weight when we consider that the petrosilex shows no 

 cleavage, that many of the layers have forms incompatible 

 with the compression theory of their origin, and that they 

 are far more rigid than anything occurring in an ordinary 

 cleaved slate. It is not easy to say under what probable 

 conditions sedimentation would give rise to these different 

 types of banding and schistosity ; but I would suggest that 

 the definiteness of the structure, the separation of the mate- 

 rials into well-defined quartzose and feldspathic layers, may 

 be partly the result of a segregating process subsequent to 

 the deposition. 



