61 



independent of the forms of the nodules, showing no tendency 

 to conform with their outlines. In short, these concretions are 

 essentially identical with those occurring in the most recent de- 

 posits of sand or clay. The concretions weather less readily 

 than the matrix, and are usually quite inconspicuous on un- 

 weathered surfaces. The larger nodules, however, are some- 

 times partially decomposed exteriorly ; and in these may be 

 discerned traces of a secondary concretionary structure, or nod- 

 ules within nodules. In this locality (near the light-house) the 

 banding of portions of the petrosilex is unusually fine and even, 

 yet much contorted ; and some of the more feldspathic layers 

 have a finely granular or brecciated appearance, precisely as if 

 the layers had been crushed in situ and reconsolidated. The 

 minute fragments appear at first sight to be quite sharp and an- 

 gular, — and so they are, as regards the fracture planes pro- 

 ducing them ; but close observation shows that most of the 

 grains are more or less rounded and concentric. In fact, they 

 are concretions, and, like those just described, they weather less 

 readily than the matrix. The concretionary structure appears 

 to be wholly wanting in none of the banded petrosilex on 

 Marblehead Neck, and it may be occasionally detected in 

 other varieties. In the petrosilex of West Dedham, west of 

 Woodland Street, Mr. F. W. Very has observed a band or bed 

 about three feet wide, strike N. 30° E., in which the spheru- 

 litic structure is well developed. The rock shows traces of 

 stratification, and consists of a greenish base enclosing occasional 

 crystals of reddish-brown feldspar, and numerous spherical con- 

 cretions of a dark-purple color, about a line in diameter, and 

 without apparent structure. The spherules are more quartzose 

 than the matrix, appearing in relief on weathered surfaces. 

 The base itself is also concretionary, resembling the "toad 

 stone." There is no visible quartz in this rock, although grains 

 of this mineral occur in the bright-green petrosilex by which 

 the stratum is bordered. On the line between Hyde Park and 

 West Roxbury, near Muddy Pond, Mr. Very has found a dirty, 

 greenish- white, stratified petrosilex with scattering spherical con- 



