260 



Scientific Geology. 



gard variolites as rather intermediate between porphyry and amygda- 

 loid,* and such I suppose to be the character of the rock under consid- 

 eration. By the term varioloid, however, I intend merely to designate 

 the external aspect of the rock ; since the mode of its formation 

 seems involved in much obscurity : but its variolous appearance none 

 can deny. 



Brochant describes wacke as " substance intermediate between ba- 

 salt and clay." This description will apply to the base of the vario- 

 loid rock under consideration. It is found in Brookline, Newton, 

 Needham, Hingham, Brighton, and Saugus. But its most impor- 

 tant varieties are found in the three latter places, and deserve a par- 

 ticular description. 



In Brighton the wacke is of a chocolate color, and quite hard. 

 The nodules are mostly rounded, and of the size of a pea j but some- 

 times they are much larger and irregular, approaching to the form 

 of veins. Compact feldspar, epidote, calcareous spar, and quartz are 

 the principal minerals of which they are composed. Sometimes the 

 external part of the nodule is compact feldspar, or calcareous spar, 

 and the central part epidote : and sometimes quartz occupies the cen- 

 ter, invested by epidote. The epidote is crystallized, although the 

 cavities are in almost every instance entirely filled. The foliated 

 structure of the feldspar, and especially of the calcareous spar, is 

 not unfrequently visible, though generally these minerals are com- 

 pact, and very hard. But the two last seem to be strangely blended, 

 as if they had been partially melted together. (Nos. 373, 377.) 



At a quarry about a mile southwest of Brighton meeting house, 

 this varioloid rock may be seen passing into conglomerate, showing 

 that it is only a variety of the graywacke formation. 



At Hingham the greater part of this rock, is of a deeper red than 

 that at Brighton ; though some of it is of a light gray. The basis 

 is harder, owing perhaps to a mixture of compact feldspar. The 

 nodules vary in size from that of a pea to that of an almond; and 

 consist of brownish red and greenish compact feldspar, with carbon- 

 ate of lime mixed with the latter ; or in separate folia. Not unfre- 

 quently the red compact feldspar encloses the green, like that in 

 Brighton. This rock is associated with a conglomerate of the gray 

 wacke formation. (No. 374.) 



At the head of Nantasket Beach, I found a rolled mass (No. 375) 



* Traite de Mineralogie, Par. T. S. Beudant, (Paris, 1830,) Vol. 1. p. 569. 



