WILSON: MEDFOJII) DIKE AREA. 
357 
grades into granite porphyries in which the ground mass becomes 
relatively diminished in amount, and the phenocrysts larger and 
more numerous. These granite porphyries are somewhat darker in 
color than the granophyr, owing to the presence of epidote and 
chlorite. The alteration of the plagioclase feldspar has resulted in 
the production of a considerable amount of calcite, as shown both 
in thin section and by eifervescence Avith dilute acid. The largest 
mass of granite porphyry occurs to the west of the large diabase dike, 
just south of the northern granite area. In the field, this rock so 
much resembles the granite, that, were it not for the fact that the 
schistose dikes which occur in the granite are cut off by it, one 
Avould be inclined to think it contemporaneous and identical with 
the latter. 
Towards the middle of the area, especially at the south crest of 
Pine hill, forming part of the top of the hill, and extending north¬ 
eastward approximately parallel with the line of contact of the 
felsites and the granite, the magma cooled more rapidly, producing 
a true rhyolite with a characteristic sub-conchoidal fracture, Avavy 
or sub-Autreous lustre, and preA^ailing gi-eenish-gray color, weather¬ 
ing nearly white. This rhyolite is in places quite porphyritic. A 
few quartz phenocrysts occur and larger numbers of orthoclase and 
plagioclase crystals, usually idiomorphic, less often fragmentary, the 
plagioclase being particularly abundant. Both the feldspars haAe 
undergone secondary changes. At the soutliAvestern foot of Pine 
hill, inclusions of Avhite quartzite Avere found included in the grano¬ 
phyr. At the northeast end of Pine hill along the line of contact 
with the granite, is a belt of breccia in Avdiich fragments of granite, 
rhyolite, and some undetermined rocks occur. 
Diabase dikes. — These acid eruptives Avere followed by a period 
of strain and fracture, as shown by their jointed condition. The 
diabase dikes are later intrusions not so much jointed. The evi¬ 
dence of strain appears in the distorted tAvinning lamellae of the 
plagioclase of the granite, and in the waA^y extinction of the quartz 
phenocrysts in the granophyr. This may also liaA-e been the time 
of conversion of the lamprophyric dikes into the present schists. 
Succeeding this period of stress or contemporaneous with it, a large 
number of diabase dikes were intruded. Of these dikes there is 
positive evidence of at least five periods, and possibly of a sixth 
period, of intrusion. These intrusions may haA’e succeeded one 
