ALTERATION OF THE FELDSPARS. 150 



crushing took place, much less intense than the first, indicated hy the 

 breaking and faulting of tourmaline prisms and the Assuring of the new 

 tourmaline mentione* I ab< >ve. This second crushing is probably to be cor- 

 related with that which broke the ottrelite prisms in the ottrelite schist. 

 Alteration of clastic Feldspars. — The alteration of detrital feldspars, prin- 

 cipally microcline, to quartz, sericite, biotite and albite is well shown. 

 This change takes place about the edges of the grains, along cleavage 

 planes emphasized by pressure, and along irregular cracks. Lines of in- 

 clusions in the clastic quartz serve to separate it from that resulting from 

 the alteration of feldspar, the latter being always clear and glassy. These 

 are commonly liquid-filled cavities with characteristic gyrating bubbles 

 of carbonate dioxide, but in both the clastic microcline and secondary 

 albite they are even more plentifully distributed. Liquid cavities in the 

 quartz may have, besides the bubble, a single prism of some indetermin- 

 able doubly refracting mineral or an apparent cube, which, however, is 

 also doubly refracting. 



One of the thin sections (number 699) has a large pebble of microcline, 

 three-fourths of an inch long, which retains its original water-worn out- 

 line, excepting at its ends, where it is partially crushed. Lines of sericite, 

 biotite and quartz traverse it regularly along emphasized cleavage planes 

 and irregularly about grains resulting from granulation. Thousands of 

 minute dots of hematite or limonite are distributed through it, and there 

 is a tendency for them to be arranged in lines normal to the basal cleav- 

 age of the feldspar. It is these that give the flesh color to the mineral 

 in the hand specimen. Other interpositions are numerous rhombs of 

 siderite, exhibiting all stages between the pure mineral and its pseudo- 

 morphous resultant, limonite. These or their limonite representatives 

 occur throughout the rock, but they are noticeably more abundant in 

 the clastic feldspars. Their distribution indicates their formation after 

 the feldspar Avas deposited in the rock, and if this be the case, there Avere 

 two periods of alteration of the microcline — one during Avhich siderite or 

 some iron-bearing carbonate was a stable alteration product under the 

 conditions of environment, and a second during which the carbonate 

 itself changed to limonite, not contemporaneously Avith the development 

 of sericite, but posterior to that change and probably after the rock came 

 under the operation of surface disintegrating influences. 



Sericite in minute hexagonal plates and prisms terminated by basal 

 planus are scattered through it and seem to have resulted from a direct 

 alteration of the feldspar without the aid of its introduction in solutions 

 from other parts of the rock. Liquid cavities are not nearly so numerous 

 as in the secondary feldspars. 



The interdependence of the different phases assumed by minerals, 



