520 PEOCEEDINGS OP THE GEOLOCICAL SOCIETY. [June 6, 



to be identical in origin with the vesicular areas of the less altered 

 greywacke beds. They occur in greater abundance than these last, 

 thus eyincing a more advanced stage of metamorphism — an inference 

 which is further strengthened by the pasty aspect of the green 

 felspathic matrix. The larger pseudo-bombs generally approach to 

 a rounded form, and increase in numbers until they often impinge, 

 and by their union form spheroidal masses of felspathic amygdaloid. 

 When the pseudo-bombs press closely, the structure of the rock be- 

 comes columnar. The passage into felspathic amygdaloid can thus 

 be distinctly traced. 



4. FelspatJiie Amygdaloid. — A general description of the amyg- 

 daloid has been given above ; but a few additional notes may be 

 added here. It shows most usually a spheroidal structure ; when 

 it becomes columnar, the columns are found to obey no order in the 

 mode of their occurrence. They seldom continue long in one direc- 

 tion, but occur at all angles to any given plane. Between this rock 

 and the green paste-rock (or nascent amygdaloid) there is, as just 

 pointed out, a close relationship. We therefore find the two rocks 

 intermingled and blended in a very confused manner. Taking a 

 general view of this part of the coast, however, there appears to be 

 a rude alternation of pseudo-ashy beds with felspathic amygdaloid.. 

 Examined in detail, no such arrangement is visible. Here may be 

 seen an irregular area of pseudo -bombs projecting from their green 

 paste, surrounded on all sides by felspathic amygdaloid; there, 

 again, may appear amorphous masses of amygdaloid completely 

 enclosed in " green paste-rock." The amygdaloid becomes in places 

 porphyritic with felspar, and passes into felspar-porphyry. Dming 

 this passage the matrix gradually loses its basic appearance, and 

 the amygdaloidal cavities decrease both in numbers and size. The 

 rock thus insensibly shades into felspar-porphyry. 



B. Felspai'-iwrphyry. 

 This is an intensely igneous-like rock. The felspathic matrix, 

 which is sometimes sparingly magnesian, is of chocolate -brown 

 and red colour, with a dull hard compact texture. It is pro- 

 fusely porphyritic, with tabular crystals of greyish felspar. Some- 

 times little spherical cavities may be detected in the body of 

 the rock ; but this is rare. At its junction with comparatively 

 unaltered greywacke, however, the felstone is in some places punc- 

 tured with numerous vesicles, which vary in size from mere pin- 

 points up to the usual small " sweet-pea " cavities. In its central 

 portions the rock is amorphous, but at its junction with greywacke 

 it sometimes shows a spheroidal structure. The phenomena exhi- 

 bited both by the felspar-porphyry and the grej'wacke on these 

 occasions deserve attention. At the foot of Balcreuchan Burn we 

 come upon an area of well-bedded greywacke's, completely sur- 

 rounded on all sides by crystalline rocks. The greywaekes are 

 often much crumpled and infinitely jointed, and some portions have 

 undergone a peculiar brecciating process which will be described in 

 the sequel. On their upturned ends rests an irregular cap of fel-^ 



