!08 RELATIVE DATES OF INTRUSIVE ROCKS. 



wMcli runs into "Wenfcworth, mica was occasionally found to accom. 

 pany tlie hornblende. The rock was rather coarsely crystalline in 

 the main body, but dykes of it, in Avhich the grain was finer, were 

 sometimes observed cutting the Ifmestone and the gneiss. These, 

 however, were never traced from any distance up to the nucleus. 



The syenite was found to be cut and penetrated by volcanic rock 

 of a porphyritic character, which is therefore of a still later date. 

 The larger masses of this porphyritic rock consist of fine-grained 

 dull reddish-buff feldspar, with which is mingled a sparing quantity 

 of fine-grained black hornblende, the mixture oonstituting a base in 

 which well-defined crystals of the same reddish feldspar, of various 

 sizes, from one-eighth to three-eighths of an inch, are thickly dis- 

 seminated ; the base is compact, presenting an impalpable grain, a 

 conchoidal fracture and a jaspoid aspect, with various colors, from 

 light to dark gray, brownish-black, and dull green. In addition to 

 crystals of red feldspar this jaspoid base often contains a multitude 

 of fragments of gneiss, greenstone and syenite, varying in size from 

 small grains to masses several feet in diameter, and these are occa- 

 sionally so abundant as to give the rock the features of a tufa. 



The principal mass of this porphyritic rock occupies a pear-shaped 

 area of about 250 acres, with the small end south, on the third and 

 fourth lots of tlie fifth and sixth ranges of Grenville, from which, on 

 the east side, a portion is projected into the second lot of the fifth 

 range. The mass is wholly surrounded by the syenite, and a large 

 part of it constitutes a mountain or group of hills, intersected by 

 one or two ravines. In about the centre of the mass, on the summit 

 of one of the hills, there exists a circular depression of about one 

 hundred yards in diameter, nearly surrou.nded by atufaceous porphy- 

 ritic rim of about thirty feet in height. In this depression — which is 

 situated in tlie sixth range, on the line between the third and fourth 

 lots, about fifteen chains from the front, — there is held a turf bog, 

 with an even surface, from which springs a growt'i of good-sized 

 greenwood trees ; and on sounding the depth of this bog with a 

 boring rod, the rock beneath was found to present the shape of a cup, 

 with the depth of twenty-five feet in the centre, so that, including 

 the rim, the depression would be about fifty feet deep, with the 

 exception of a break down to the level of the bog, on the east side. 

 The natui^e of tlie rock, and the difficulty of accounting for the 

 depression by any mode of wearing, gives to it in some degree the air 

 of a small volcanic crater. But if it were such, it mu?t represent 

 only the deeply-seated base of the crater, as the evidence which is 



