GEOLOGY OF FRANKLIN COUNTY 113 



While the one is the ordinary green brown hornblende of the 

 plutonic rocks, the other is a peculiar brown hornblende which 

 seems identical with that recently described by Van Horn in a 

 Tiornblende-gabbro near Ivrea, 1 It has the same strong pleochro- 

 isin, a — faint yellow, b — reddish brown and c — orange-brown, with 

 absorption b ^ c > a, and the other optical characters are in close 

 agreement. Like that also, it is associated with a green spinel 

 'which is only found included in the hornblende. 



The distribution of this mineral is peculiar. While it occurs 

 to a slight extent in the granular material mingled with the 

 pyroxenes, garnet and feldspar which compose it, it has in the 

 main an exclusive tendency, occurring in bunches in which the 

 individuals are usually of larger size than the ordinary granular 

 material and have in part good crystal outlines. Sometimes they 

 are clustered around a magnetite crystal or crystals, sometimes 

 no magnetite appears in the cluster. In either case feldspar forms 

 their outer 'boundary. The phenomena are precisely such as char- 

 acterize what are known as reaction (or, better, as corrosion) rims. 

 The mineral seems to be of secondary origin in these gabbros and 

 to have been formed as a result of metamorphism from the inter- 

 action of feldspar and magnetite, or more rarely augite. A few 

 minute inclusions in the primary feldspars may be, themselves, 

 primary or have resulted from the same process that produced 

 the garnet inclusions. In the Ivrea rock, on the contrary, this 

 mineral is certainly primary, but it does not form corrosion rims 

 and holds the minute inclusions which are found in the other 

 primary minerals and which are lacking in the Adirondack 

 mineral. In the nests in which magnetite is lacking, it is judged 

 to have been there originally and to have been completely used 

 up in the process. 



Most of the green hornblende is also secondary, but it makes 

 more show in the granular material than the brown does. The 

 latter is the usual hornblende in the bosses, the green in the dikes. 

 Where the metamorphism has been considerable, only the green 

 appears. Beyond question primary green hornblende occurred 

 in some of these rocks, which must have originally been in part 



lVan Horn, F. R. Tscher. Min. Petr. Mitt, band 17, heft 5. 



