Hyatt,] 224 TJanuary 19, 



representing the horizontal layers. The changes in the majority of 

 cases follow this pattern, so that the included pebble becomes re- 

 duced much faster in its vertical than its horizontal diameter, thus 

 assuming a more elliptical and flatter form. The whole series of 

 bands, which are thus seen to arise from above and below simulta- 

 neously, approximate more and more to a horizontal line in ap- 

 proaching the centre of each pebble until they actually do meet on 

 one common level. 



An infinite number of pebbles arranged with the longer axes in the 

 planes of stratification, and undergoing such changes as these just 

 described, would, by the intersection of their laminas, form the more 

 or less concentric or continuous and irregular bands which are to be 

 found in what are called banded porphyries. 



Another form of porphyry is also found on the Neck in which the 

 pebbles seem to be absolutely flattened out, and then to fuse or run 

 together at their extremities, forming dark continuous streaks or 

 bands. The precise mode of the formation of this kind I did not 

 succeed in following out, and in fact attempted, with regard to the 

 others, nothing more than what could be accomplished by the most 

 direct visual observations unassisted by chemical analyses. Never- 

 theless some curious facts can be observed in the merely mechanical 

 phenomena attending these changes. It is excesdingly interesting to 

 note that so great changes, as those described, could take place, and 

 in a mass which must have been sufficiently plastic to permit of a 

 continuous chemical reaction between the elements of the pebbles and 

 those of the surrounding matrix, and yet not so plastic as to alter 

 the contour of the pebbles. Also, that different kinds of rock, 

 felsites, crystalline and banded porphyries, were produced essentially 

 from the same conglomerate, but that in all of these, while the chemi- 

 cal and physical changes in the pebbles differed, the general facts 

 remained, that in all cases the loose materials of the matrix exhibi- 

 ted the metamorphosis first, and the pebbles more slowly, the 

 changes in the latter proceeding concentrically always from without 

 inward. This would seem to indicate that the plasticity of the ma- 

 trix, if it was plastic, communicated itself very slowly, if at all, to the 

 contained pebbles. 



Mr. L. S. Burbank made some remarks on the Conglom- 

 erate of Harvard, Mass. 



This formation is of very limited extent, covering an area of about 

 two miles in length by four or five hundred feet in width. The 



