BASIC VOLCANIOS OF HEMLOCK FORMATION. 129 



of chlorite, iron oxide, rutile, and quartz is also greater. The quartz is in 

 very fine grains. The presence of the feldspar can only be determined 

 with difficulty, and usually only on the edges of the sections, as the large 

 amount of chlorite in the center conceals it. The textures caused by the 

 feldspar and the amygdules still indicate the original character of such 

 extremely altered stages. Figs. A and B, PI. XXX, illustrate such a rock, 

 showing the secondary porphyritic muscovite and calcite, and also the 

 original amygdaloidal character. 



A still further stage of alteration gives a rock whose groundmass is 

 composed of the finest-grained quartz and of grains and needles of brown 

 rutile (anatase?). In this lie rhombohedra of ferruginous calcite, plates of 

 muscovite, and irregular flakes of chlorite. The rock is macroscopically 

 gTay, hard, and quartzitic, has a ferruginous, brown, weathered crust, 

 effervesces with cold HCl, and yet shows its volcanic character by the 

 numerous beautiful amygdules. These stand out on the surface like 

 pebbles in a conglomerate. In some cases the weathering brings out the 

 concentric character of the filling very nicely. For example, some may 

 be seen in which the core is quartzitic, and is standing sun-ounded by a 

 ring-like depression, showing by difference in the weathering the difi^erent 

 character of the mineral filling. Under the microscope the only amyg- 

 dules which happened to be cut by the section were found to be filled with 

 fine-grained quartz, with chlorite in automorphic flakes at the center of the 

 amygdules, and lying in the quartzitic mass. The macroscopical appear- 

 ance of some of the amygdules shows that just the reverse condition also 

 exists, that is, that quartz forms the centers and chlorite surrounds it. 



The extreme stage of such an alteration is a rock which shows no 

 amygdules macroscopically or microscopically, but is otherwise like the 

 groundmass of the above last-described rock. It would be impossible to 

 determine the original character of such a rock except by its association. 



The extremes of texture obtained in the alteration processes are, on 

 the one hand, a porphyry with eruptive groundmass and secondary pheno- 

 crysts; on the other, a porphyritic schist, in which all elements are secondary. 

 These extremes are connected by gradation varieties, in some of which the 

 calcite and muscovite approach more closely to the size of the elements 

 composing the groundmass, and which consequently approach the ordinary 

 schists in structure. 



MON XXXVI 9 • 



