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way between Castle Rock and the northern end, and also occurs 

 near the light-house. It is the rock, pebbles of which are 

 enclosed in the schistose petrosilex, as already described. On 

 a preceding page I have mentioned this rock as transitional 

 between petrosilex and granite. It appears at first view to 

 contain no quartz, but a closer examination reveals very many 

 minute grains of this mineral in some portions of the rock, 

 with occasionally a speck or small scale of hornblendic mate- 

 rial. As regards texture, at least, the gradation between this 

 crystalline petrosilex containing quartz in visible grains and the 

 coarsest granite on the Neck is nearly perfect ; and I cannot 

 refrain from expressing the opinion that Prof. Hyatt's view 

 of the transitional nature of this variety of petrosilex is well 

 sustained by the facts. This variety and the preceding seldom 

 occur in contact with the breccia, and have not been observed 

 to hold pebbles. Yet it is probable that these, no less than the 

 compact and schistose varieties, have been subject to softening 

 and extravasation. 



Still another variety of petrosilex is known to occur in the 

 Marblehead region. This consists of a compact, jaspery base 

 enclosing feldspar crystals and grains of vitreous quartz, — the 

 so-called quartz-porphyry or elvanite. In a small mass of this 

 rock on the west side of Castle Rock the base is dark-gray, the 

 feldspar crystals are inconspicuous, and the quartz grains small 

 and numerous. A band of this rock about one hundred feet 

 wide crosses Lowell's Island toward the southern end, in a 

 N. E.-S. W. direction. This is a handsome rock; the base is 

 dark purple, weathering red; and the quartz, which appears to 

 be in the form of crystals, is more abundant than the crystalline 

 feldsj)ar. Of course such a rock must be highly acidic ; and 

 this is proved by analysis, the specimen analyzed yielding 

 82.4 per cent, of silica, which indicates nearly fifty per 

 cent, of free quartz. Part of the exotic petrosilex on South 

 Gooseberry Island belongs here ; it is of a dull-black color. On 

 the north-west shore of Marblehead, opposite the city of Salem, 

 and in other parts of this town, there are masses of the elvan- 



