314 Darton and Keith — Dikes of Felsophyre and 



and in the plagioclase are portions of the ground mass of greater 

 or less size and also of biotite and magnetite. In this section 

 decomposition by weathering has proceeded to great lengths, 

 and the smaller feldspars in the groundmass, and to some extent 



the phenocrysts, have decomposed with a production of consid- 

 erable chlorite, calcite, and limonite. The groundmass consists 

 of the finely felted, holocrystalline aggregate of plagioelase, 

 orthoclase, biotite, augite, magnetite, and calcite with a few 

 crystals of quartz. 



General Nature of the Dikes. 



The general distribution of these acid and basic dike rocks in 

 an east and west belt has been already mentioned. Beyond 

 that rude arrangement it is difficult to decipher anything 

 systematic. In the basic rocks, while they all have the same 

 physical appearance, contain the same minerals, and bear a 

 decided family resemblance to each other, there are three dis- 

 tinct types of texture. These are the basaltic, the diabasic, and 

 the porphyritic textures. The porphyries appear upon the 

 west side of Jack mountain, one of which has been described 

 by Diller and another in these pages. The rock with the 

 ophitic texture of diabase appears in section 15, in one of the 

 southwestern outcrops. The remainder are intermediate in 

 appearance between these, sometimes inclining to one type, 

 sometimes to the other. In these also the feldspars have fre- 

 quently a decided fluidal arrangement, both through the body 

 of the section in a general way, and around the larger crystals. 

 The same arrangement appears to some extent in the porphyritic 

 type. Section No. 16, taken from Sounding Knob, displays 

 *the most clearly fluidal arrangement. In this area of the basalt 

 it is evident, from the appearance of the rock mass upon the 

 ground, that it was an old volcanic neck and was produced by 

 an injection of considerable height. Aside from these features, 

 there seems to be no arrangement discernible. 



As the chemical analysis shows, these basic eruptives are of 

 the same class as those which appear in direct connection with 

 the Jura-trias sediments. Under the microscope, also, they 

 have the same appearance and contain the same minerals. 

 Inasmuch as these Jura-triassic eruptives in some cases pass 

 from the Jura-trias sediments into the Silurian sediments, the 

 occurrences here are plainly in the same line. They offer, 

 therefore, nothing especially novel. The acid eruptives, how- 

 ever, constitute an entirely new class of such phenomena. 

 They have not been observed, within the writer's knowledge, at 



