BASIC DIKES IN AECHEAN. 47 



in color. The constituents of the schistose eruptives, arranged according 

 to their relative importance, are biotite, hornblende, chlorite, quartz, feld- 

 spar (?), calcite, epidote, iron oxide, sphene, and muscovite. 



The clear limpid grains which form the groundmass are undoubtedly 

 for the most part quartz. No satisfactory results were obtained in the tests 

 for feldspar, but it is highl}^ probable that some is associated with the 

 quartz. Dark chocolate-brown to light-brown biotite is almost an invari- 

 able constituent. In some cases it is accompanied by a little chlorite, which 

 appears not to have been derived from the biotite. In a few rare instances 

 biotite is absent altogether, chlorite taking its place. The biotite and chlo- 

 rite are usually found between the quartz grains. They have a parallel 

 arrangement, and this gives the rock its schistosity. Biotite and epidote 

 are found included in the grains of quartz of the groundmass. Muscovite 

 is rarely present, but when found is in medium-sized automorphic plates. 

 Ragged pieces of ore, either ilmenite or titaniferous magnetite, and sphene, 

 secondary to these, are found in almost all specimens, and in a few instances 

 iron pyrites was observed. Calcite is invariably present in irregular, fairly 

 large grains, almost equaling the quartz in quantity. Epidote is found in 

 large quantity, both in crystals and in irregular grains, the crystals occurring 

 among the bunches of biotite and included in the grains of quartz. The 

 large amount of epidote in association with the calcite seems to point to the 

 very basic character of the feldspar of the original rock. 



A bluish-green hornblende is rather frequently associated with the 

 mica. In rocks in which the hornblende predominates mica is always pres- 

 ent, but the reverse is not true, the most micaceous rocks being entirely free 

 from the hornblendic component. 



The hornblende is found in large prismatic individuals without terminal 

 faces. This mineral contains some of the other constituents of the rock in 

 which it is found, such as quartz, epidote, and more rarely iron oxides. 

 The interspaces between the hornblende crystals are filled with irregular 

 biotite flakes and with grains of quartz, epidote, and iron oxide. This 

 hornblende is apparently one of the last, if not the last, mineral to develop. 

 The hornblendic rocks are not nearly so schistose as the micaceous ones. 



The secondary origin of the hornblende is clearly shown in one of the 

 sections which is traversed by a fissure; the hornblende can be seen extend- 

 ing into, and in places crossing, this fissure. The other minerals are 



