NORITE 227 



foliation (gneissic) in places, but frequently they appear entirely massive 

 in structure. In the more highly feldspathic fades of the rock (gabbro 

 proper) the light and dark colored minerals are so distributed with refer- 

 ence to each other as to impart a decidedly speckled appearance to the 

 rock. Variation in the proportion of the light and dark minerals is 

 observed, but as a rule the dark ones are in excess. 



The recognizable minerals in hand specimens of the gabbro are pyrox- 

 ene, hornblende, feldspar, and sometimes biotite. In the Floyd County 

 rock sulphides range up to 50 per cent of the total rock-mass. Pyroxene 

 is more abundant than hornblende, the latter mineral apparently failing 

 in some specimens. A distinct cliallage parting may frequently be seen 

 in specimens with the aid of a pocket lens. Like the syenite and granite 

 types previously described, the weathered surfaces of the gabbro show 

 pitting from the removal of pyroxene and feldspar, leaving hornblende 

 and some of the, lesser more resistant minerals standing in relief. 



MICROSCOPIC DESCRIPTION 



Study of thin sections of the gabbro under the microscope shows the 

 principal minerals to be cliallage, hornblende, hypersthene, occasional 

 biotite, and calcic plagioclase. Minor constituents are quartz, apatite, 

 titanite, magnetite, and sulphides (pyrite and pyrrhotite). Pyroxene, as 

 augite (diallage) and hypersthene, the former usually in excess, is the 

 most important constituent. It is developed in stout allotriomorphic 

 grains, containing the common platy forms of brown inclusions, which 

 ar.e more abundant in the augite than in the hypersthene, and, in addi- 

 tion, inclusions of the minor constituents. The augite is usually very 

 fresh, but hypersthene is partially or completely altered, the manner of 

 alteration being identical with that of the syenite, yielding a fibrous pale 

 green amphibole, accompanied by the separation of iron oxide in extreme 

 cases, and frequently considerable calcite. 



The feldspar is completely altered in all thin-sections of the horn- 

 blende gabbro studied, but from the character of the alteration products 

 it is inferred that probably two varieties are present. Kaolin is the most 

 frequent alteration product of the feldspathic constituent, with which is 

 associated some muscovite. A reasonably fresh specimen of the biotite- 

 rich gabbro (pyroxenite) studied contained but little feldspar, the prin- 

 cipal variety of which was andesine-labradorite, showing well developed 

 albite and sometimes pericline twinning. An appreciable amount of 

 orthoclase and some quartz were present. 



Hornblende is an important mineral of the rock, but is not so abundant 

 as pyroxene and is not so uniformly distributed. The optical properties 



