ACID VOLGANICS OF HEMLOCK FOEMATION. 81 



acid lavas which are now likewise holocrystalline, but which owe this 

 character to the devitrification of an original glassy base, supposing them 

 in their original vitreous condition to have corresponded to the modern 

 hyalorhyolite-porphyries. 



EHYOLITE-PORPHYRY. 



The rhyolite-porphyries on fresh fracture are dark grayish-blue to black. 

 From this they grade with advancing alterations through chocolate brown 

 to purplish. The weathered surface varies from white to reddish. The 

 weathering has in one case brought out very well the fluxion banding of 

 the rock. Their texture is very pronouncedly porphyritic. The quartz and 

 feldspar crystals stand out plainly from the groundmass, which is usually 

 dense with a somewhat resinous luster. The porphyritic quartzes average 

 perhaps the size of a small pea, and hence are macroscopically very plainly 

 visible. They frequently stand out on the weathered surface and show 

 their cryt-tal forms, and in other cases we see the angular cavities out of 

 which they have fallen, like the kernel from the nut. 



Under the microscope the rocks are seen to be typical rhyolite-por- 

 phyries. The phenocrysts are chiefly corroded dihexahedi-al crystals of 

 quartz. Crystals of plagioclase and orthoclase are less common. These 

 lie in a very fine-grained holocrystalline groundmass, composed largely of 

 feldspar and quartz, with some zircon in small crystals, and here and there 

 magnetite. 



These are presumed to be the original constituents of the groundmass. 

 Associated with them are considerable quantities of secondary chlorite, 

 epidote, biotite, muscovite, calcite, and reddish to brown alteration products 

 of the magnetite. Included in the groundmass are here and there oval 

 areas of finely crystalline secondary quartz, probably filling former 

 amygdaloidal cavities. 



In thin section the crystal contours of the quartz phenocrysts are 

 more or less rounded, with here and there embayments of the groundmass 

 projecting into them. The crystal form is, however, always clearly 

 marked. In some cases the individuals have been broken before the cool- 

 ing of the magma, the fragments of an individual, though now separated, 

 being seen to conform to one another. That they have been subjected to 

 pressure is shown by the undulatory extinction and also by the separation 

 MON xxxvi 6 



