Pirsson and Washington — Geology of Med Hill, JV. H. 263 



logic relations may be properly mentioned here. The chief 

 type composing the mass is a coarse-grained light-gray feld- 

 spar rock of a granite-like texture rather thickly dotted with 

 stout black hornblendes about \ inch long. This is found in 

 and about the central saddle, on the south peak, and at the 

 steep slopes at its foot, it forms the north spur and occurs else- 

 where. The north peak, however, towards its top has a some- 

 what different character, the rock is finer-grained, the feldspars 

 thin tabular and more or less arranged in parallel position 

 producing a trachytoid texture and somewhat schistose frac- 

 ture. In the exposures near the Home farm in the west 

 valley places may be found where these types merge into one 

 another, without any contact, within a few inches or less, and 

 blocks in the stone walls at this place show the same thing. 

 On the other hand, an exposure in the road below this house 

 seems to indicate the finer trachytoid type as a dike in the 

 coarser eugranitic one though the evidence is not clear. Again 

 the rock in the quarry pit opposite the Home farmhouse, 

 which, as will be seen on examining the illustration and map, is 

 near the center of the exposed mass, is richer in nephelite than 

 in any other locality ; the trachytoid type of the north (and 

 highest peak) is very poor in it, while at the contact with the 

 gneiss the nephelite dies out entirely and is replaced by quartz. 

 These facts appear to indicate that as a whole the mass of 

 syenite is of low silica content at the center ; that this rises as 

 one goes away from it and becomes quickly very marked near 

 the contact, the texture remaining the same but the grain 

 diminishing ; that the same is true in a vertical direction, but 

 that here the rock assumes a different and trachytoid texture. 

 The possible reasons for this change in silica will be discussed 

 later. The vertical variation is of interest geologically since 

 it shows the influence of the former roof, long since removed 

 by erosion, and it is clear that, if the endomorphic effect of 

 this upper cover is still to be seen, the erosion of the syenite 

 mass in a vertical direction has not been excessive and suggests 

 thus, in a vague way it is true, the original upper limit of the 

 intrusion and the amount of erosion the district has suffered 

 since it took place. 



Salic Dikes. — The study of the area has shown that, in 

 addition to those already mentioned as occurring at the eastern 

 contact, a considerable number of other dikes are exposed 

 which are genetically related to the syenite intrusion. As is 

 so commonly the case in examples of this kind, they are of two 

 classes, aplitic'and lamprophyric, or in field usage, felsites and 

 traps, sometimes porphyritic and sometimes not. In the 

 quantitative classification the former are liparase and the latter 

 camptonase or in Rosenbusch's system aplite in its narrower 



