69 



are decidedly flat, and, what is more to the point, the degree 

 of flatness is nearly uniform in them all. Transitional stages 

 between these attenuated layers and wholly unflattened pebbles 

 are entirely wanting. There are no slightly distorted pebbles 

 to mark the beginning of the flattening process, or to show 

 whether it was initiated by compression or by the peculiar mode 

 of deposition invoked by Prof. Hyatt. The angular pebbles in 

 the * * flattened pebble " petrosilex exhibit no distortion what- 

 ever, and they are far more distinctly outlined, less intimately 

 united with the enclosing rock or paste, than is common with 

 the pebbles in the breccia. Between the schistose petrosilex 

 and the evenly banded petrosilex, also, intermediate degrees 

 of flattening are not to be found, for the types of structure 

 represented by PI. 1, figs. 4 and 5, although truly transitional 

 if regarded as original, as the direct result of sedimentation, 

 could scarcely be the outcome of continued flattening and 

 coalescence of the layers in fig. 6. But the most conclu- 

 sive evidence against the pebble origin of the layers in the 

 schistose petrosilex is found in the nature of the layers 

 themselves. In the first place the forms of the layers are in 

 most cases such as could not result from the flattening of 

 any ordinary pebbles, being ragged, fringed, or deeply bifur- 

 cate on the margins. Secondly, the great physical, and prob- 

 ably also chemical, uniformity of these limited sheets would, 

 to say the least, be very exceptional in a conglomerate 

 rock, and is not found in any of the undoubted breccia on 

 Marblehead Neck. The fragments entering into the composi- 

 tion of a breccia, however, are less likely to be of a multiform 

 character than the pebbles of a pudding-stone, since they are 

 usually deposited nearer the parent ledges. Therefore, we will 

 grant the possible existence of a breccia composed of materials 

 as homogeneous as these schistose layers and confine our atten- 

 tion to the inquiry whether this schistose structure could, by 

 any conceivable process, be developed in it. Here, I think, 

 the answer should be in the negative. As already observed, the 

 great majority of these layers are very minute, often containing 



