226 Bulletin of Laboratories of Deiiison University. [Voi xii 



A section cut parallel to the direction of the pencils pre- 

 sents a surface streaked with long, somewhat irregular, though 

 roughly parallel, black lines, more or less perfect dendritic or 

 fern-like forms (Figs. 2 and 3). I was shown recently a large 

 slab of the rock collected from one of the outcrops since my 

 examination in the summer of 1903, which, for perfection and 

 delicacy of tracery in fern like forms, was beautiful beyond de- 

 scription. The black streaks or pencils which characterize the 

 rock are composed of a staining of the oxides of manganese 

 and iron. 



The rock is cryptocrystalline in texture, breaking with a 

 conchoidal fracture, and is intensely hard and tough. Minute 

 quartz crystals of doubly terminated pyramidal faces are dis- 

 tributed through the rock at irregular wide intervals. These 

 are nowhere abundant in the rock, but they are always present 

 to some extent, and consist both of the light-colored and dark, 

 smoky, vitreous quartz. Indeed, unless carefully examined, 

 the rock would ordinarily be pronounced non-porphyritic in 

 texture, so small and scattering are the porphyritically devel- 

 oped quartzes. Megascopically, porphyritic texture is nowhere 

 particularly emphasized in the rock, but its slight development 

 is best seen on a weathered surface of the stone, where the un- 

 altered quartz crystals, though few in number and widely scat- 

 tered, contrast more strongly with the weathered surface and 

 appear more conspicuous than in the fresh rock. Feldspars 

 are also porphyritically developed, as described below, though 

 the phenocrysts are difficult of differentiation in hand specimens 

 of the rock. 



MICROSCOPICAL DE.SCRIPTION. 



In thin sections the rock consists of a holocrystalline 

 groundmass and scattered small porphyritic crj'stals. Flow- 

 structure is not exhibited in the groundmass, and the pheno- 

 crysts indicate no orientation with respect to each other. The 

 groundmass is micro-granitic in structure, though some sections 

 show much of the micro-granophyric structure, with an irregu- 

 lar radial, spherulitic, structure developed in greater or less 



