1876.] 223 [Hyatt. 



contained pebbles. This is shown by some of the masses of the truly- 

 crystalline porphyry in which the pebbles have entirely disappeared 

 in fractured surfaces, but show the outlines of their uncompressed 

 forms upon the external weathered faces. 



That the conglomerate porphyries cannot have been derived from 

 the adjacent masses of banded and crystalline porphyries is inferred 

 from the fact that the materials of the conglomerate are not identi- 

 cal. The pebbles contained in them are evidently derived from some 

 older porphyries, and are quite distinct. Besides this, the traces of 

 pebbles may be seen upon the weathered surfaces of the crystalline 

 porphyries and felsites, and their transformations traced back to their 

 original condition in the conglomerate in many localities. 



The change of the pebbles into more or less lenticular masses, 

 streaks, or bands, in the formation of the banded porphyries, is 

 also very interesting. In this case the re-arrangement of the 

 conglomerate, itself recomposed from older banded porphyries, takes 

 place in a similar manner, but with certain distinctive character- 

 istics. The material of the pebble is seen to be re-arranged, as 

 it were, by the action of the matrix, into alternate bands of dark 

 colored porphyry and white feldspar, marked here and there with 

 imperfect crystals, the remnants of the centres of pebbles which 

 have otherwise entirely disappeared. This re-arrangement pro- 

 ceeds from without, so that the pebble eventually becomes a lentic- 

 ular mass arranged in alternate lamina?. This would seem to be the 

 direct product of pressure upon the mass, which would naturally 

 produce the lenticular form, and lead, especially if a moderate 

 amount of heat were applied, to the production of bands of feldspar. 



But if we examine the form which the lamina? of the coarse, peb- 

 ply matrix assume during deposition, we find that this lenticular 

 form can be explained without bringing in the aid of pressure. 

 These layers can be traced in many specimens. They are concave 

 around the bases of the larger pebbles, straight or horizontal only at 

 the middle part or zone around the centre, and become decidedly 

 convex as they are heaped up on the upper half of the inclosed 

 mass. The changes which take place first affect the lowermost and 

 uppermost layers of the matrix, converting them into bands of feld- 

 spar and dark amorphous porphyry. These form a lenticular figure 

 surrounding the pebble and the zone of intermediate horizontal lay- 

 ers, exactly as the lines of the eyelids surround the ball of the eye, 

 supposing the corners of that organ to be filled with solid matter 



