524 PEOCEELINGS OF THE GEOLOGICAL SOCIETY. [JunO 6, 



merely a stage in the metamorphism. We may suppose the por- 

 phyry spheroids, during the period of their outgrowth, to have been 

 constantly surrounded with a paste of felspathic matter, in which 

 the felspar crystals were generated. Thus, as the metamorphism 

 proceeded, case after case would become obliterated and absorbed 

 into the body of the rock, while, at the same time, new cases of 

 felspathic stuff would continue to form on the fresh surface of each 

 outgrowing spheroid. 



The felspar-porphyry is only very sparingly amygdaloidal. The 

 surface of the spheroidal portions is, indeed, often abundantly pitted ; 

 but the body of the rock rarely if ever shows any amygdaloidal 

 cavities ; nevertheless a passage from amygdaloid into felspar-por- 

 phyry may occasionally be traced. Some greywackes must there- 

 fore have passed through the stages of " green paste-rock " and 

 amygdaloid before they at last became metamorphosed into felspar- 

 porphyry. But the gradation to be observed from granular grey- 

 wacke into finely crystalline felstone, and from that into porphyritic 

 felstone, shows us that porphyry may sometimes be produced directly 

 from aqueous rocks without an intermediate vesicular stage. AU will 

 depend upon the composition of the rocks undergoing alteration. 

 If they are highly basic, a vesicular condition will probably precede 

 the production of porphyry ; if, on the other hand, the beds are 

 less basic, a previous vesicular stage does not seem necessary to the 

 formation of porphyritic felstone. 



C. Brecciiforin RocJcs. 



The anomalous brecciiform beds which are now to be described 

 are of sporadic occurrence. EoUowed along the general strike of 

 the strata they quicldy die out, even when they have attained a 

 considerable thickness. They are typically developed on the coast 

 near Bennane Head, and the beautiful hill of Knockdolian is made 

 up of them. 'No description Can convey an adequate notion of their 

 character. Knockdolian Hill consists of a mass of brecciiform rock, 

 entirely amorphous and unstratified, except in one or two places, 

 where, however, the appearance of bedding may be due to jointing. 

 The brecciiform fragments are of all sizes, from mere dust up to 

 stones twice the size of a walnut ; but much larger fragments are 

 common enough : while most are of angular shapes, many have a 

 somewhat rounded and subangular aspect, often bearing a striking 

 resemblance to the small lapilli of a volcanic ash. They consist of 

 fine-grained and compact felspathic rocks. Matrix is usually want- 

 ing, but is sometimes present in meagre quantity : when it occurs 

 abundantly, the rock has all the appearance of a scoriaceous ash ; 

 and on such occasions it sometimes becomes difficult to distinguish 

 it from the green paste-rock. The composition of the matrix is 

 chiefly felspathic, with an admixture of crystalline carbonate of lime 

 in some places. But, as just remarked, matrix is for the most part 

 wanting, and the stones are huddled together in the direst con- 

 fusion. In the rock of Knockdolian Hill one or two rounded stones 

 were obtained, whose smoothed surfaces told their story of former 



