PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 215 



Aporhyolyte.] 



"The porphyritic feldspars in the thin section are found to be either or both of 

 orthoclase and oligoclase. TJieij arc <tlirni/x tnrl'nl from decomposition, and are more 

 commonly red-staine<l ilnni not. They have always crystalline outlines, or, when they 

 have been eaten into by the still fluid matrix, as is not seldom found to have been 

 the case, at least the remnants of such outlines." 



The stained orthoclases are very distinct from these anorthoclases. They are 

 a conspicuous feature of most of the rock of the Great Palisades. The anorthoclases 

 are glassy and never stained by secondary changes. They are found to include par- 

 ticles of the glassy magma, now reddened by iron oxide, and thus to be opaque in 

 small spots, but these inclusions are very distinctly different from the general stain- 

 ing or turbidness ascribed to the orthoclases by Irving. The determination of 

 "adularia" by the writer was made by Wallaston's goniometer on the surface angles 

 of some isolated crystals, and on some thin sections made for microscopic study. At 

 the same time the fact that porphyritic quartz was also present in this rock .was not 

 detected, the evidence of the surface angles being applied inferentially to all the 

 translucent crystals. 



The reddened matrix shows distinctly a fluidal structure, viewed at large, and 

 under the microscope it is apparent in the "streaming" which crowds round the 

 corners of the crystals and which spreads out into fan-shaped and streaked areas. 

 The whole section is occupied by such evidences of flowage. It is more evident here 

 than in the bulk of the Palisades, and may be due to a second fusion superinduced 

 by the intrusion of the great basic sill (?) of later date (?) which underlies the cliff, 

 and which separated the Palisades from the similar rock appearing about a mile 

 below the mouth of Baptism river. 



There is, however, a more minute structure pervading the matrix of these crystals, 

 which is to be ascribed to the contracting effect of the period of cooling. The rock 

 being glassy or rnicrofelsitic, it seems to have taken on various curved fissuring, by 

 reason of which the peculiar ferritic circles and loops were afforded opportunity to 

 locate themselves. There are also straight fissures, indicated now by the needle- 

 shaped ferritic lines. There is sometimes a grouping of angular areas, each included 

 between four or more straight reddened cracks, in each of which is a series of curled 

 and non-connecting red lines. The curling is mainly in individual groups or areas, 

 but sometimes the curves are crossed by the straight boundaries; rarely the straight 

 lines themselves part and blend with the curved lines. 



The second form of minutely polarizing substance in this rock, which gives the 

 megascopic fludial or banded aspect, has also a unique microscopic fibrous structure. 

 It shows but rarely, if ever, a distinctly fluidal arrangement, yet when, as frequently 

 happens, one of the porphyritic crystals lies in this substance, there is sometimes a 



