REPORT OF THE DIRECTOR I (J 1 4 45 



places is the zone of dark silicates, particularly of hornblende, which 

 intervenes between the ore and the gneiss. These zones are some- 

 times composed of hornblende alone, a dark green strongly pleo- 

 chroic variety, but usually contain more or less quartz and feldspar 

 as well as biotite and magnetite. A conspicuous secondary product 

 is epidote which is rarely absent. Pegmatite may take the place of 

 the hornblende zones along the borders, forming the wall for a 

 greater or less distance. The pegmatite is probably of more than 

 one period of intrusion. For the most part it bounds the deposit 

 without cutting or affecting in any way the magnetite ; but in a few 

 places there is pegmatite that seems to be later than the ore, as 

 instanced in some Putnam county mines that are mentioned by 

 Stewart in the paper already quoted. 



A modification of this type of occurrence is illustrated by those 

 deposits in which pegmatite is in direct contact with the magnetite 

 on one or both walls throughout its extent, although the general 

 country rock is gneiss or mixed gneiss and pegmatite. Under this 

 condition the magnetites are more coarsely textured and lack the 

 layered arrangement characteristic of the bodies bordered by gneiss 

 alone. In the Forest of Dean mine, which is the principal example 

 of this type, the magnetite forms a long pod in contrast with the 

 simple tabular form of the deposits already described. The cross- 

 section of this body is heart-shaped, the two lobes somewhat drawn 

 out and directed upward following the steep dip of the country 

 gneiss. The pitch is 20 northeast. The ore is of shotlike texture 

 and maintains a uniform percentage of iron so as to be practically 

 all of shipping grade. The two lobes are separated by a horse of 

 pegmatitic granite which extends from the surface to the bottom of 

 the workings now 3000 feet on the incline. The same rock is also 

 seen along the border of the main ore mass in many places, and 

 inclusions of it are found in the ore itself. So far as the writer 

 has been able to learn there is no evidence to connect the latter 

 with dike intrusions ; rather they appear to be floating masses with 

 the ore. 



A third class consists of disseminated magnetites within peg- 

 matitic granite. The ore is a mixture of coarse crystalline magne- 

 tite with the ordinary pegmatite minerals and may carry from a 

 few per cent to 40 per cent of iron. These bodies of course have no 

 particular shape or trend of themselves but depend on the local 

 development of the pegmatite mass which may take the form of a 

 lens, a dike or a rounded bosslike body in the gneiss. They are 

 free of pyrite and afford an ore easy to concentrate, although they 



