Varioloid Wacke. 



261 



of the varioloid rock, whose base is brownish gray, and the nodules 

 a greenish compact feldspar. 



In Needham this rock has a somewhat slaty structure, is hard, and 

 contains distinct crystals of feldspar of a light green color. (No. 378.) 

 But as the basis is obviously wacke, exhaling an argillaceous odor, 

 I can hardly persuade myself to place it among the porphyries. 

 Suppose this Needham rock were to be subject to a degree of heat 

 sufficient to fuse the feldspar, without essentially altering the wacke, 

 I enquire whether the result would not be a rock very similar to 

 some varieties that have been described as varioloid wacke. And 

 may not this have have been the mode in which some of that rock 

 was produced ? 



The most remarkable of the varioloid rocks which I am describing, 

 occurs at Saugus. Near the center of the place, and surrounded by 

 granite, we find a rock, forming a hill one or two hundred feet high, 

 composed of a basis of green wacke and white compact feldspar, 

 with an occasional mixture of carbonate of lime. The nodules are 

 rarely so large as a bullet ; more commonly about the size of small 

 peas, and in some parts of the rock, so very numerous that it seems 

 hardly possible they could have been infiltrated into cavities pre- 

 viously made. (No. 372.) The basis is a pleasant green. I saw 

 no conglomerate or other variety of graywacke in the vicinity. 



It is obvious from the preceding descriptions, that in some instan- 

 ces — particularly at Brighton — the nodules of this varioloid rock 

 must have been at least partially formed by the infiltration of earths 

 from a watery solution : but it would seem that this was only a part 

 of the process. For it is difficult to conceive how such minerals as 

 compact feldspar and carbonate of lime could have been deposited in 

 a compact form from a watery solution ; since they crystallize with 

 so much readiness. It seems to" me that we must call in the agency 

 of heat, after the infiltration took place, by which the crystals might 

 be converted into a compact mass, and all the cavities be filled, as 

 they are in almost every instance : and if we suppose granite, sienite, 

 &c. to have had an igneous origin, we can be at no loss to provide 

 for the requisite heat. I had been rather disposed to regard much 

 of this rock as an example of the solid concretionary structure, es- 

 pecially that at Saugus. But the occasional evidence of infiltration 

 led me to abandon that hypothesis. If the one hinted at above 

 is more satisfactory, I shall be gratified. The subject is certainly 

 involved in much obscurity. 



