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cretions, the largest of which are about one-fifth of an inch in 

 diameter. They have a radiate structure, and appear to consist 

 of crystalline quartz. The greenish tinge of this rock is due 

 to a soft, brownish-green mineral resembling serpentine, which 

 forms layers in the rock. It is, however, probably aluminous 

 rather than magnesian. The same observer also reports the 

 occurrence of concretions in an otherwise structureless petro- 

 silex in Hyde Park Village. The petrosilex ridge between Pine 

 Garden and Calvary Cemetery, in Hyde Park, — especially 

 towards the southern end, — affords the best example which I 

 have yet observed of the concretionary structure in the petrosilex. 

 This is a white, banded variety of petrosilex, and the concre- 

 tions are usually spherical, varying in size from a pin-head to 

 five inches in diameter. Occasionally they are so numerous 

 that the rock is almost entirely composed of them, and the 

 weathered surface has the aspect of a pudding-stone. The nod- 

 ules are commonly purplish, weathering white, and showing 

 but faint indications of concentricity. In some limited masses 

 of the petrosilex of a bright-red color, the concretions are small, 

 white, and distinctly stellate. On the east side of the ridge, 

 toward Back Street, the spherical concretions are mostly want- 

 ing, and are replaced by small, purple layers, which, though of 

 very irregular shapes and sizes, always present botryoidal sur- 

 faces and a concentric structure. They coincide with the band- 

 ing of the petrosilex ; are rarely more than an inch thick, but 

 often two feet or more in length. Abundant concretions have 

 been observed in a portion of the petrosilex forming the rounded 

 north-east corner of the Blue Hill area of petrosilex. They 

 are mostly small, generally spherical and radiate, but some- 

 times elongated and concentric. Small fragments of a feld- 

 spathic rock containing concretions have been found near the 

 trilobite quarry in Braintree. This rock has not been ob- 

 served in situ. It holds scattering feldspar crystals, which are 

 much fewer and several times larger than the rounded nodules ; 

 each crystal, however, is enveloped in a whitish layer, giving it 

 a concentric appearance. The foregoing are all the instances 



