Tol. 55.] INTO DIABASE AT SOEEL POINT (NOKTHEEN JEESET). 435 



IV. Rocks containing Deeived Mateeial. 

 (a) Granite with Material absorbed from the Diabase. 



In general terms the diabase occupies the upper part of the cliff, 

 the red granite the lower. Here and there, in a position between 

 the two, a rock is found intermediate in petrographical character, 

 though varying in its composition as traced from point to point. 

 It corresponds to the porphyritic variety of granite with absorbed 

 material and fragments, described in § II. When the amount of 

 ferromagnesian minerals is small, the rock approaches the normal 

 granite in appearance very closely, but as the basic constituents 

 increase in ptoportion so, of course, the closeness of this resemblance 

 is lost. The rock is remarkable for its quartz and large prominent 

 orthoclases : finer material, with much biotite and hornblende, 

 occupies au interstitial position. The more rapid weathering of 

 the latter causes knobs of rounded felspar and quartz to stand out 

 on the rough surface of the rock, giving a distinctive character 

 which at once catches the eye. A difference in colour, increasing 

 in depth with increasing basic material, can be seen readily enough 

 in the rocks themselves, especially from a short distance, but it 

 cannot be definitely said that the more basic is nearer the diabase. 

 The rocks at this point are so intermingled on the broken and rugged 

 crags of the cliff that it is safe to speak onlj^ of general relations. 

 Yery numerous inclusions of diabase and altered diabase are found 

 in this intermediate rock ; often the edges of these fragments are 

 more distinct than might have been supposed, seeming to show 

 that the absorption of basic material stopped abruptly. All stages 

 can be seen within a short distance, from an inclusion apparently 

 unaltered, to a patch the origin of which could not have been guessed 

 had the transitional steps been wanting. 



The intermediate rock most nearly approaching the granite in 

 character offers but few noteworthy points in a thin section. The 

 large orthoclases, often with a microperthitic structure, show in 

 many cases a zone of quartz-grains near their peripheries. Their 

 inner edges are set very nearly in a straight line, their outer 

 elongated and more irregular. They seldom appear to be con- 

 nected optically with quartz-grains external to the felspar-crystal, 

 and doubtless mark merely a phase of consolidation during which 

 silica was in excess. Externally to this zone the felspar is con- 

 tinued irregularly among adjacent crystals. The parts of finer 

 grain interstitial to the larger quartzes and felspars contain a 

 considerable proportion of quartz in rounded grains, felspars occa- 

 sionally striped, and biotite with some hornblende in small irregular 

 flakes. 



A second section, representing a specimen from within a few 

 yards of the first, and containing more basic material than it, 

 shows an increased proportion of hornblende and mica, the former 

 predominating. Both minerals are larger and better formed than 

 in the first slide. In one case a plagioclase has been greatly 



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