IGNEOUS ROCKS 313 



rock, with accessory quartz, hiotite, hornblende, magnetite, apatite, titan- 

 ite, pyrite, pyrrhotite, garnet, and orthoclase. Exclusively decomposi- 

 tion minerals are actinolite, chlorite, and serpentine. Quartz is very 

 variable in amount, ranging from to 30 per cent. 



The pyroxenic constituent may be exclusively hypersthene, chiefly 

 hypersthene, or less frequently, chiefly augite or exclusively augite or 

 diallage. It constitutes from 10 to 40 per cent of the rock. 



The plagioclastic constituent is labradorite or labradorite-bytownite^ 

 and varies from 5 to 50 per cent of the rock. Between the pyroxene, 

 whether hypersthene or augite, and the labradorite there occur reaction- 

 ary peripheral bands of garnets. Between the garnets and the pyroxene 

 there is a narrow zone of quartz and hornblende. Hypersthene and the 

 anorthite molecule will produce hornblende, quartz, and common garnet 

 (an isomorphous mixture of grossular, almadine, and pyrope). In the 

 case of the monoclinic pyroxene the feldspar molecule is not needed 

 and lime is liberated. These garnet rims are the most striking petro- 

 graphic features of the gabbro (see plates 57, 58, and 59). 



Wherever the gabbro has been subjected to pressure, as along the 

 periphery of the intrusive mass, pyroxene is replaced, chiefly by green 

 hornblende and subordinately by biotite. Such a margin of hornblende- 

 gabbro is so marked a feature of the Cecil County gabbro mass that it 

 can be mapped as a separate formation. A more or less schistose struct- 

 ure accompanies the development of these two minerals. Along the con- 

 tacts hornblende also accompanies biotite as a constituent of the gneiss. 

 This contact phenomenon increases the difficulty of separating the two 

 formations. 



The gabbro is associated with and, through decrease in feldspar, grades 

 into pyroxenite or, with the addition of olivine, into peridotite. 



The youngest material into which the gabbro intrudes is the Ordovician 

 mica-schist. The gabbro is therefore post-Ordovician in age. Its rela- 

 tions to the granite are such as to indicate that the granite, which is also 

 post-Ordovician, is the earlier intrusive. 



By the previous surveys it has been included with the " Laurentian 

 gneiss " or syenite. 



XLII— Bum.. Geoi.. Roc. Am., Vol. 16, 1904 



