2 20 THOMAS L. WATSON 



areas from manganese and iron oxides, forming the dark spots and 

 pencils in the hand specimens, occur. The thin sections are char- 

 acterized by the complete absence of ferro-magnesian minerals. 



Feldspar is apparently in largest quantity, and is composed of 

 both potash and plagioclase species. Occasional grains of micro- 

 cline are recognized which show the characteristic microcline twin- 

 ning. The unstriated feldspar grains so strongly resemble quartz 

 that it is impossible in many cases to distinguish them without the 

 application of optical tests. Optical tests show the plagioclase to be 

 albite^a circumstance entirely confirmed by the chemical analysis 

 of the rock given below in the table of analyses under I, in which only 

 the barest trace of lime is indicated, with soda in large amount and in 

 excess of the potash. Some of the plagioclase exhibits polysynthetic 

 twinning according to the albite law, and at times assumes lath- 

 shaped forms. The feldspar substance is generally fresh, but the 

 individual grains are usually rendered dark by, abundant, closely 

 crowded, minute, dark, dust-like particles, the identity of which could 

 not be made out. 



■ Quartz is of the usual kind and presents no noteworthy features, 

 further than its occurrence in small mosaics of interlocking grains, 

 which occupy at times distinct areas in some of the thin sections. 



Light- colored mica, tinged a faint yellow, is very generally distrib- 

 uted through the sections, in the form of irregular minute shreds, 

 groups, and aggregated masses, the folia of which are at times imper- 

 fectly arranged radially about a common center. A part, at least, 

 of the mica is clearly secondary, while some of it is yet doubtful as 

 to origin, whether primary or secondary. Its general appearance 

 and association in the sections might very well indicate secondary 

 formation for all of it. 



Phenocrysts of both quartz and feldspar occur in well-developed 

 idiomorphic forms, usually in rectangular and squarish cross-sections. 

 In the thin sections studied phenocrysts of feldspar are more abundant 

 than quartz; and while the porphyritic texture is poorly developed in 

 the hand specimens, it is very pronounced in the thin sections. The 

 quartz phenocrysts show irregular fractures free from impurities, 

 strained shadows, and occasionally inclose grains of feldspar. The 

 porphyritic feldspars show in part broadly twinned bands of plagio- 



