312 Darton and Keith — Dikes of Felsophyre and 



bending, as if due to some movement after its formation. In 

 the groundmass there is a small amount of secondary chlorite 

 in extremely small particles, and an even smaller amount of 

 muscovite, both apparently due to decomposition by weather- 

 ing. 



No. 17. — This rock has the typical aspect of the acid erup- 

 tives. It consists of an extremely tine groundmass of a light 

 gray or dove color, through which are distributed large pheno- 

 crysts of feldspar and smaller ones of biotite. The feldspar 

 phenocrysts do not differ so greatly from the color of the 

 groundmass as to be conspicuous. Dark scales of the biotite, 

 however, stand out very prominently. The feldspar pheno- 

 crysts are in part bright and clear and in part kaolinized, 

 thus separating the feldspars into two groups. The biotites 

 are in flat, hexagonal crystals and are very sharp and clear. 



No features appear in this section which were not observed 

 in No. 18 except a slight tendency to parallelism of the feld- 

 spar crystals in the groundmass, particularly around the outlines 

 of the phenocrysts. This is quite marked in several cases. The 

 phenocrysts consist of biotite and plagioclase. In a few cases 

 doubtful orthociase appears, in which the mineral was nearly 

 removed in the making of the section. The amount of mag- 

 netite is decidedly less than usual, and some fine chlorite, 

 apparently derived from original biotite in the groundmass, 

 marks the progress of decomposition by weathering. A little 

 of the groundmass is included in the phenocrysts, but usually 

 their outlines are sharp and perfect. 



No. 19. — This specimen, like the preceding one, is badly 

 weathered. The general gray or dirty white color of the rock 

 is varied by the limonite stains at the surface and in the pheno- 

 crysts, and by the prominent black biotite phenocrysts. Many 

 of the large feldspar crystals have been entirely removed, leaving 

 only the cavities which they once filled. The biotite crystals 

 appear as fresh and unaltered as in the other specimens 

 examined. The phenocrysts in general are slightly smaller 

 than in the other specimens. This is true, perhaps, of the 

 biotite in greater degree than of the feldspars. The proportion 

 of the rock which the phenocrysts make up is somewhat less 

 than usual, the biotite crystals in particular being plainly fewer. 

 One or two doubtful cases of augite were also observed. 



In this slide the proportion in size between the pheno- 

 crysts and the individuals of the groundmass is considerably 

 changed. The phenocrysts are much smaller, while the ground- 

 mass is so much coarser that most of its individuals appear dis- 

 tinctly as separate crystals. The two series, however, are still 

 greatly different in size and the rock is accordingly porphyri- 



