CONSTITUTION OF THE MICACEOUS GNEISS 427 



amount estimated at about 65 per cent of the volume of the rock. Of 

 these, the quartz grains, amounting to 40 per cent of the rock, are an- 

 gular and clear, inclosing a few plates of hematite, biotite, brown zircon, 

 needles of fibrolite, and sheets of minute fluid cavities. No clastic forms 

 or evidences of secondary enlargement were distinguished. The feld- 

 spar commonly displa3^s polysynthetic twinning after the albite and 

 sometimes the pericline law. Wavy extinction rarely occurs. 



A few small grains of orthoclase also are found. The schist plane of 

 the rock is strongly marked by parallel disposal of the longer axes of the 

 grains of quartz and feldspar, and still more strongly by the plates of 

 the other minerals. Biotite occurs, to the amount of about 18 per cent, 

 in reddish brown plates, with the usual strong absorption and })leochro- 

 ism, the latter rarely blue to red. Muscovite, to about 7 per cent, is 

 generally inclosed within the biotite in colorless plates, which sometimes 

 reveal an obscure fine lineation on the plane of basal sections. Silli- 

 manite occurs in a few fragmentary, colorless plates, with cross-partings, 

 high relief, and parallel extinction. Some show incipient alteration b}^ 

 variation in their bright interference colors. Fibrolite is also common 

 in fibrous bundles intermixed with finely granular quartz. Garnet is 

 distributed in pale reddish dodecahedra, mostly six-sided in cross-sec- 

 tion. None display zonal structure, but in many irregular cloudy 

 bands (strain borders) occur along the margin, which are found to be 

 anisotrope between the cross nicols, while the center and greater part 

 of the crystal remains dark. A little zircon occurs, imbedded in the 

 quartz and muscovite, as minute brownish crystals of high relief and 

 strong double refraction, and sometimes in radiating groups near the 

 edges of the mica scales. A few irregular particles, shining plates and 

 elongated scales of bluish black opaque hematite are also present. 



Amphibole Schists 



mode of occurrence 



These occur intercalated among all the gneisses of the island, and, 

 though often found in proximity to the limestone, are never inclosed 

 within it nor in contact. Besides these, some observers have referred to 

 the occurrence of pyroxenic schists, concerning which little information 

 is on record. 



VARIETIES 



The following varieties are commonly distributed : 



Fine-grained quartz diorite schist, and hornblende schist, passing into 

 massive hornblendite, into biotite schist, or becoming more or less 

 epidotic and thus altered into epidosite. 



