BASIC YOLCAN1C8 OF HEMLOCK FORMATION, 127 



further stage of change tlie feldspars are partly altered to a granular 

 aggregate of various minerals. In ordinary light the textures of igneous 

 rocks are still preserved, but in polarized light none are seen, with the 

 exception of amygdules which may be present. In some cases even these 

 are obliterated, and the original nature of the rock can onl}" be determined 

 from its mode of occurrence and its association. 



Fm'ther changes may produce rocks which consist practically of calcite, 

 and may be nearly white. 



Agaua, from these basic rocks there may be produced in extreme cases, 

 by a process of silicification, a rock which consists practically of pure silica. 



Description of some phases of alteration. As illuStratiug SOUie CaSCS iu wllicll the 



same alteration products, but in different proportions and arrangement, give 

 rocks differing very essentially, there are given the following brief descrip- 

 tions of some of the rocks studied. 



The flow structure was noted as being- exceedingly well develojjed in 

 the microlitic rocks, and in some of them the production of amphibole 

 needles and chlorite flakes has taken place parallel with the long direction 

 of the feldspar microlites (the flowage direction), thus developing, in com- 

 bination with the unaltered microlites, a well-marked schistosity. Tlie 

 feldspars are still fairly well preserved. 



In another case the feldspar microlites have become completely sericit- 

 ized, the interspaces between them being occupied by epidote, chlorite, 

 and iron oxide. The preservation of the feldspar shapes, showing in ordi- 

 nary light the igneous texture of the rock, gives the only clue to its original 

 nature. (Figs. A and B, PI. XXVIII.) In- some of the basalts the feldspar 

 is replaced chiefly by epidote-zoisite, and, as in the above case, such rocks 

 show their igneous character only when examined in ordinary light or by 

 uncrossed nicols. (Figs. A and B, PL XXIX.) 



In still other rocks calcite is very abundant. Its occurrence in por- 

 phyritic rhombohedra and scalenohedra was mentioned in the description of 

 some of the rocks. These porphyritic calcites have thvis far been found only 

 in the fine-grained microlitic types of groundmass, the coarser ophitic rocks 

 having it only in the usual granular aggregates. Muscovite, occurring in 

 large porphyritic plates, confoi-ms in occuiTence to the calcite. When 

 musco^dte is present, calcite is found associated with it in every case, 

 though the calcite may occur alone, and this latter is also by far the more 



