ZONAL WEATHERING OF A HORNBLENDE 201 



outside, much-altered portion, and of the part of the weathered zone 

 adjacent to the fresh material. A specimen from an entirely fresh 

 portion of the same dike was analyzed for a comparative study. 



The fresh rock is finely grained, holocrystalline, nearly black in 

 color, with small white feldspar crystals evenly distributed through 



Fig. i.— Specimen showing zone of weathering Xh- Circle shows approximately 

 the part from which the slide shown in Fig. 3 was taken. 



the mass, which is dominantly hornblende. Small garnets, irregu- 

 larly distributed, and small crystals of pyrite are present. Micro- 

 scopically, the rock is found to consist of hornblende (65 per cent), 

 which may be secondary, labradorite (32 per cent), with small 

 amounts of orthoclase and augite, and accessory magnetite, pyrite, 

 titanite, and apatite. One of the typical slides is shown in Fig. 2. 

 The rock is a hornblende gabbro. 



