GEOLOGY OF FRANKLIN COUNTY 107 



The most common rock is a quartzaugite-syenite, composed 

 ■essentially of microperthitic feldspar (orthoclase and albite or 

 •oligoclase), augite, hypersthene or bronzite and quartz. Horn- 

 blende is almost always present as well, and in a considerable 

 portion of the rock comes to exceed the augite in quantity. In 

 these hornblendic varieties the hypersthene is usually absent, and 

 ordinarily some biotite appears. Quartz is usually present, as 

 it is in the augite syenite. The two varieties shade into one an- 

 other with facility. 



Accessory minerals are zircon, apatite, magnetite, pyrite, garnet, 

 •titanite, allanite, and oligoclase feldspar. Garnet occurs but 

 sparingly and, so far as noted, only as corrosion rims around 

 magnetite. Its scarcity is in strong contrast to its abundance in 

 The gabbro rocks. On the other hand titanite, rare in the gabbros 

 and anorthosites, is here quite characteristic, while the rare allan- 

 ite does not occur in them at all so far as noted. 



The green augite and the bronzite (or hypersthene more rarely) 

 are often minutely intergrown in thin parallel plates, and, while 

 the minerals themselves and the hornblende as well are closely 

 like the same minerals in the gabbro rocks, these intergrowths 

 have not been observed in them and seem quite characteristic of 

 the augite syenites. 



The greater part of the feldspar consists of the minute inter- 

 ^rowth of orthoclase and an acid plagioclase to which the name 

 "" microperthite " is applied. The plagioclase is usually albite. A 

 little oligoclase is usually present in addition to the microperthite. 

 No microperthites with oligoclase cores, similar to those described 

 by Smyth in the augite syenite at Diana, in Lewis county, have 

 been seen in Franklin county. 



Quartz seems to be present in all the syenites of the county, 

 though in the more basic varieties it is in only slight amount and 

 wholly in the form of small inclusions in the feldspars, such as 

 are frequently found in the feldspars of the entire group. But, in 

 addition, in the more acid members is much coarse quartz, which 

 in a large part of the rock assumes the elongated, spindle form 

 already alluded to. 



