ACID IGNEOUS ROOKS. 117 



GRANITE-PORPHYRY. 



Granite-porphyry occurs in the northwestern margin, about Diamond 

 Hill and half a mile northwest of Arnolds Mills. 



This latter area affords a tolerably fresh rock, characterized by the 

 presence of large grains of quartz and orthoclase, with microscopic garnet 

 as a rare accompaniment. The quartz occurs in rounded and angular 

 grains. The angulation of the quartz is probably in part due to a move- 

 ment of the magma after partial cooling. The quartz also shows embay- 

 ments of the groundmass. It is further characterized by the usually 

 observed fluid inclusions, with gas bubbles, which move about in the cavi- 

 ties. Some of these bubbles simply change their position with reference 

 to gravity when the slide is turned up or down; others keep up a contin- 

 ual oscillatory movement, which is independent of the accidental jarrings 

 due to the manipulation of the stage of the microscope. Magnetite or 

 ilmenite is present. Apparently the iron ores of the granite-porphyries of 

 this district tend to form aggregates having the external grouping of den- 

 dritic minerals. Chlorite is present in the groundmass, as are also small 

 crystals of hornblende. Garnet occurs in isotropic sections with a trian- 

 gular outline, a pinkish tinge in plain light, and with traces of cleavages. 

 Concerning the interpretation of these porphyry stocks, see page 155. 



OTHER ROCKS. 



Farther south are bosses of hornblende rock which break through the 

 Blackstone series This is a fine-gTained, dark-blue rock, evidently of 

 igneous origin. Under the microscope it shows considerable alteration. 

 Hornblende occurs in idiomorphic crystals, affording basal sections. There 

 is a great deal of what appears to be secondary hornblende, shown by 

 its lack of crystalline outline. Chlorite is abundantly present in scaly, 

 felty aggregates. Muscovite exists, probably as an alteration product of 

 one of the original feldspars. Calcite occurs in the rock, and is probably 

 derived from a lime-soda feldspar. The idiomorphic character of the horn- 

 blende crystals, where this character is preserved, the nature of the second- 

 ary products, and the fact that quartz is present, probably as a secondary 

 product, lead to the opinion that this rock was originally a diorite charac- 

 terized by little or no free quartz, the quartz now visible having been 

 derived by the breaking up of the bisilicates. 



