Plate 6 



W. J. Miller, photo 



Upper figure. Photomicrograph of thin section of gabbro from the top 

 of Hackensack mountain. Magnified 15 diameters. The central mineral, 

 with good cleavages nearly at right angles, is diallage which is almost 

 completely surrounded by a zone of hornblende (darker colored). As 

 seen under the microscope the rim is very clearly defined. The light gray 

 minerals are l)asic plagioclase. The dial)asic texture of the rock is well 

 shown. 



Lower figure. Photomicrograph of a thin section of diabase from the 

 dike one and one-half miles southeast of Johnsburg. Magnified 15 diameters. 

 The long, slender prisms with transverse cracks are augite ; the smaller, 

 more numerous, prisms arranged in more or loss sheaf like bundles arc basic 

 plagioclase; the black areas are glassy groundmass ; and the four small, 

 white rounded areas represent quartz, probably of secondary origin. 



