286 FINLAY 



mass. They are often less than an inch in width. To the un- 

 aided eye they present the appearance of dense basalt except as 

 being porphyritic by reason of the shining black crystals of 

 hornblende which they contain. The contacts with the enclos- 

 ing nephelite syenite are very sharp. Even in the smallest 

 dikes near the walls little included pieces of the nephelite 

 rock may at times be found. These dikes are almost always 

 quite fresh, but cavities filled with secondary minerals are 

 met. 



Microscopic Description. — Under the microscope this variety 

 of the camptonite appears as a holocrystalline porphyritic rock 

 with marked fluidal arrangement. Hornblende in two genera- 

 tions makes up over four fifths of the whole. Augite is present 

 in phenocrysts, perhaps five per cent, of the total rock. Feld- 

 spar in minute areas is packed away in the interstices of the 

 ground-mass. Apatite and magnetite are prominent. The 

 ground-mass shows pronounced flow structure. The grain is 

 much finer near the contact with the enclosing nephelite syenite. 



The phenocrysts are chiefly brown basaltic hornblende, with 

 no noticeable effects of alteration. The basal sections of this 

 mineral are sometimes .5 mm. in diameter. They are of the 

 usual six-sided forms and generally give indications of zonal 

 structure. Long prismatic individuals are often dull green at 

 the center with an outer rim of the brown hornblende. In other 

 crystals this order is reversed. The angle of extinction is low, 

 not over 9°. Pleochroism is confined to shades of brown. 

 Twins are common. Apatite and magnetite are the usual in- 

 cluded minerals and rims of the latter due to magmatic resorp- 

 tion are sometimes seen. (See PI. XII, fig. 2.) 



Phenocrysts of augite are much rarer. This mineral is violet 

 gray in color and scarcely pleochroic. Its crystal outlines are 

 never perfect for a surrounding rim of serpentinic alteration pro- 

 ducts is invariably present. The smaller augites of the ground- 

 mass are entirely altered to serpentine. Almost the whole of 

 the ground-mass however is made up of hornblende and inter- 

 stitial feldspar. Magnetite grains at times . 1 5 mm. across and 

 large perfect crystals of apatite occur sporadically. 



