60 



concretionary structure is really wide-spread, and is probably 

 in every case an original structural feature of the petrosilex. 

 Extended observation has convinced me that this structure, 

 more or less perfectly developed, characterizes most of the 

 petrosilex of this belt, especially the banded variety. It can 

 usually be detected only on weathered surfaces, but is some- 

 times so pronounced as to be observable in the mass of the 

 rock. The concretions are all small, and, as in the "toad 

 stone," are confined to the feldspathic material. The nucleus 

 can usually be seen, indicating the concentric structure ; but 

 I have failed to detect the greenish layer. The resemblance, 

 though not complete, is probably sufficient to establish the 

 essential identity of the two rocks. 



A small mass (probably an erratic) of petrosilex breccia, 

 found on Marblehead Neck by Prof. Hyatt, contains pebbles of 

 a banded petrosilex similar to that in Newbury, which are 

 characterized by a nodular structure, differing somewhat, but 

 probably not essentially, from that described above. In 

 the banded petrosilex of Marblehead Neck, also, I have ob- 

 served abundant traces of a similar structure. On the north- 

 ern end of the Neck, near the light-house, the stratified petro- 

 silex contains numerous intercalated layers, varying in thickness 

 from an inch or less to a foot, which are largely, and, in some 

 cases wholly, composed of concretions or nodules, The con- 

 cretions are of all sizes, from those requiring a magnifier to 

 make them visible, to two inches in diameter. The smaller 

 concretions are all spherical, but those exceeding half an inch 

 in diameter are more or less flattened on the under sicie, and the 

 largest are disc-shaped. There is little appearance of separa- 

 tion according to sizes, the large and small being usually inter- 

 mingled. When crowded the larger nodules give the upper 

 surface of the layer a botryoidal aspect. A concentric structure 

 is sometimes observable in the concretions ; but usually they 

 appear quite structureless, save in this : the regular banding or 

 stratification of the petrosilex is continuous through the nodules. 

 The banding, both in the nodules and outside of them, is quite 



