310 Darton and Keith — Dikes of Felsophyre and 



rock and appear most prominently upon the lighter weathered 

 surfaces. 



In the thin section appears the ordinary ophitic arrangement 

 of the Piedmont diabases. The texture is rather finer than 

 usual in these rocks. The minerals consist of a great mass of 

 feldspar intergrown with and enclosing numerous magnetite 

 crystals and patches. Many crystals of olivine of somewhat 

 porphyritic appearance interrupt the ophitic structure, and 

 around their borders the feldspar laths have a tendency to 

 parallelism. Secondary decomposition has proceeded to con- 

 siderable extent in these olivines, resulting in patches and 

 cracks filled with chlorite and leucoxene. In many cases 

 limonite is deposited in the same situations. Frequent small 

 individuals of augite appear between the feldspar laths, and 

 small, patchy individuals of calcite are also to be seen. These 

 do not usually have crystalline outlines. One crystal of augite 

 of somewhat prophyritic appearance is intergrown with magnet- 

 ite. 



No. 16. — Few macroscopic characters are well defined in this 

 rock. It is of a dark, gray color, weathering to a lighter gray 

 upon exposure. The feldspars of the groundmass, which are 

 extremely fine and invisible upon fresh surfaces, are brought 

 out in the lighter gray portions affected by the weather. Por- 

 phyritic crystals of augite are sparsely distributed through the 

 rock, and patchy phenocrysts of feldspar appear here and there 

 in the groundmass. In one layer these have a tendency to an 

 amygdaloidal appearance. A few scales of biotite are also to 

 be detected. The presence of olivine is shown in the partly 

 weathered portions by a greenish color of the rock, and also a 

 few phenocrysts of the same mineral are sparingly distributed. 



In this section the rock is holocrystalline and displays no 

 evidence of glass. The plagioclase laths are comparatively 

 small and their arrangement is more nearly parallel than 

 ophitic. The body of the rock is mainly composed of these 

 crystals. Intermingled with them are fine crystals of augite, 

 olivine, and magnetite, the latter in unusual abundance. No 

 additional features of interest appear in this section. 



Acid Eruptives. 



In the hand sj)ecimen these eruptives have a decided light 

 gray color, which passes, in the weathered specimens, to white 

 or yellowish white, according to the amount of decomposition 

 of the feldspars. The texture is in general fine. The ground- 

 mass is light gray in color ; from this stand out the white 

 feldspar and the dark biotite and augite phenocrysts. Decay 



