1866.] J. GEIKIE CAKlilCK, AYESHIEE. 523 



The amorphous green paste-rock with its pseudo-bombs, it was 

 said, is only a more altered greywacke, the pseudo-bombs corre- 

 sponding to the amygdaloidal areas of the distinctly bedded and less 

 altered wackes. The whole appearance of this peculiar rock be- 

 tokens a greater degree of metamorphism. . The bedding has gone ; 

 the amygdaloidal areas or pseudo-bombs are in greater abundance ; 

 and their matrix is harder, and sometimes even semicrystalline. 

 The manner in which these pseudo-bombs become aggregated, 

 so as to form by their union masses of felspathic amygdaloid, has 

 been described above. 



It may be objected to this that the green paste-rock, instead of 

 exhibiting an arrested stage of metamorphism, may be merely the 

 decomposing wacke of an igneous rock. But the pseudo-bombs do 

 not exfoliate like the spheroidal portions of a decomposing trap ; 

 their weathered crust (seldom thicker than a penny) is itself quite 

 hard. The composition of the green paste differs considerably from 

 that of the pseudo-bombs : the latter show apparently no alkaline 

 matter, save in their amygdaloidal cavities; while the former is 

 abundantly charged with it, and in its unweathered portions it has 

 the same composition. It is difficult to believe that a rock so con- 

 stituted could have resulted from the decomposition of a felspathic 

 trap. If the pseudo-bombs were the only solid parts remaining of 

 such a cleconiposed and weathered trap, we should expect to find 

 them shading off into the surrounding decayed portions of the bed. 

 But nothing of this is apparent. Their junction with the green 

 paste is bo sharply defined, that no one who sees that rock can for a 

 moment suppose it to have resulted from the decomposition of a 

 felspathic amygdaloid having the same mineralogical character as 

 the pseudo-bombs. Bearing in mind the occurrence of distinct 

 amygdaloidal areas in well-bedded and granular greywacke, I am 

 convinced that in the green paste-rock we have just the same 

 phenomena, but in a higher state of development. 



The felspar-porphyry must be regarded as the maximum stage of 

 metamorphism exhibited by the felspathic rocks of the district. The 

 passage from granular sedimentary beds into this very igneous-like 

 rock can be so well traced as to leave no doubt of its metamorphic 

 origin ; while the peculiar junction of its spheroidal portions with 

 comparatively unaltered greywacke is quite unlike any junction of 

 igneous and aqueous rocks. The manner in Avhich the shells or 

 cases are moulded round the spheroids of porphyry appears to indi- 

 cate that they were once in a sufficiently pasty condition to allow of 

 their being pressed outwards by the increasing spheroids of felstone. 

 But it would almost seem as if their want of alkaline matter, by 

 rendering them less easily assailable, had stopped the further pro- 

 gress of the metamorphic action in their direction. It is certainly 

 very remarkable that, although we find the fels])ar crystals of the 

 porphyry well developed on the surfaces of the spheroids, and even 

 coating the concave side of the shell- cases, we yet canrot detect 

 here a passage from the one rock into the other. Upon the whole, 

 it is perhaps preferable to regard the cases of hard reddish rock as 



