308 Barton and Keith — Dikes of FeJsoj)hyre and 



The percentage of silica in the typical specimen of acid rock 

 No. 18 is seen to be entirely characteristic of granite. The 

 same may also be said of the other chemical constituents. 

 Silica only appears as free quartz in the groundmass, being 

 entirely absent from the phenocrysts. The amount of it in the 

 groundmass, moreover, is relatively small. Jn this feature of 

 inconspicuous quartz, taken in connection with the trachytic 

 aspect of the rock in the hand specimen, these acid eruptives 

 resemble the syenite-trachyte family. The minerals, however, 

 and the chemical proportions show that it is of the granite 

 family, and in its name the porphyritic aspect should be recog- 

 nized for a complete description. The grain of the groundmass 

 is micro-crystalline, but since it has not the true granitic aspect, 

 the term u granite " or its allied terms cannot be properly 

 applied. Neither does the groundmass contain glassy portions 

 or any matter which can be definitely stated to be felsitic, 

 owing to the alterations of the rock due to weathering. As a 

 whole, however, the rock is more nearly a porphyritic felsite 

 than anything else, and should, therefore, be designated as a 

 line " felsophyre." 



The basic eruptives exhibit no changes, either in chemical 

 composition or in component minerals, from the usual types of 

 the Jura-trias basic rocks. 



A brief description of each of the specimens and thin sections 

 follows, from which the variations of the rocks can be seen. 

 The localities from which these sections are taken are shown 

 upon the accompanying map, fig. 1 (p. 305), by corresponding 

 numbers. 



Basic Rocks. 



No. 5. — This rock in the hand specimen consists of a rather 

 coarsely crystalline aggregate in which porphyritic individuals 

 of olivine and augite are prominent. The groundmass is line 

 and of a dark gray or black color. On the weathered surfaces 

 decay has brought out the crystals of feldspar slightly. The 

 olivines are green and yellowish brown in color and appear 

 unaltered. The greenish color in some of the olivine is due 

 perhaps to small replacement by chlorite. The augites are of 

 a dark gray or blackish gray color and show fresh cleavage 

 faces. Aside from these two porphyritic minerals and the 

 small feldspars upon the weathered surface, no other minerals 

 cau be detected in the hand specimen. 



Under the microscope this rock differs greatly in appearance 

 from the following sections and has a very decided porphyritic 

 character. The phenocrysts are composed of augite and 



