‘ 
246 H. P. CUSHING—SYENITE-PORPHYRY DIKES. 
usual strong absorption. It is mostly somewhat altered and is some- 
times completely gone to chlorite, magnetite forming at the same time. 
In the 4 most acidic dikes ferromagnesian silicates are absent, except 
for a few minute shreds of chlorite present in the groundmass which 
probably represents altered biotite. 
Hornblende.—This mineral occurs in 4 of the dikes, but in only 2 as the 
principal ferromagnesian silicate. It has an extinction angle of at least 
12 degrees, and is strongly pleochroic in orange-brown and dark bluish 
green tones. It isso vaguely bounded and so altered as to prevent exact 
determinations or to give satisfactory interference figures, so its optical 
scheme is uncertain, though in character it seems like arfvedsonite. 
Augite.—Kakle reports augite, almost completely altered. in the dike 
he describes. No mineral that can be decisively referred to augite 
occurs in my slides, but 2 of the dikes show a few grains of a gray-lilae 
mineral which is thought to be titaniferous augite, and a third shows 
granular epidote which may have resulted from the alteration of augite. 
A little undoubted titanite occurs in one slide. 
Quartz.—In most of the dikes quartz is present, and in about half of 
them in considerable amount, constituting from’3 to 15 per cent of the 
rock. It only occurs in the groundmass and is wholly allotriomorphic. 
The more granitic dikes have a miarolitic tendency, and contain some 
minute quartz and clear orthoclase which seem secondary, but in no case 
is the amount great. 
Tron oxides.— All the dikes contain either magnetite or specular hem- 
atite, commonly with sharply idiomorphic boundaries. Their amount is 
greatest in the more acid dikes and decreases as the ferromagnesian sili- 
cates increase; the specular iron occurs in the former rather than the latter, 
Its non-magnetic character and red streak leave no doubt of its nature. 
Considerable secondary magnetite (from biotite) and hematite stain 
are found in many instances, 
Pyrite.—Pyrite is sparingly present, in rather large idiomorphic crys- 
tals. It has been noted in two of the dikes and may occur in others. 
Apatite.—All the dikes contain apatite in slight quantity in the usual 
minute needles, as inclusions in the other constituents. In dike 9 alone 
does it appear in any quantity. Here it is in larger, irregularly bounded 
fragments and forms an important item in the rock’s make-up. Quali- 
tative tests, after separation by Thoulet solution, give such strong reac- 
tions for chlorine as to make it doubtful if any fluorine is to be found in it. 
Muscovite-—This mineral is present in small shreds in 2 of the more 
acidic of the dikes, in both cases accompanied by calcite which has often 
good idiomorphic boundaries. As the feldspars are quite fresh and dark 
silicates practically lacking, these minerals can not be alteration products 
of the rock itself, though the calcite certainly and the muscovite prob- 
