312 A. GEIKIE ON THE SUPPOSED 



grains of what appears to be magnetite are dispersed through the 



Of the clastic fragments still recognizable, quartz is most conspi- 

 cuous. It presents the usual characters of sand-grains, with lines 

 of fluid inclusions. These grains usually show sharp borders. 



Among the schists of the volcanic group there occur small 

 fragments of felsite, with less sharply defined margins, which I 

 cannot doubt were originally volcanic lapilli. I conjecture also that 

 the ovoid aggregates of chlorite may represent the augite and olivine 

 of the more basic fragments. The schists, it will be remembered, 

 are interstratified among tuffs in which felsite and porphyrite 

 lapilli are quite distinct. 



The schists intercalated among the sandstones and shales above 

 the conglomerate, present essentially the same structure and the same 

 ingredients as those among the tuffs below. So close, indeed, is the 

 resemblance, that I am inclined to look upon these schists as having 

 been originally fine tuffs. They contain, as might be supposed, a 

 larger proportion of quartz-sand ; but their sericitic constituent is 

 well developed. 



One further microscopic character may be referred to. A slide 

 taken from a band of fine schist, among the tuffs beyond the bridge 

 over the Allan to the north of the Board Schools, shows an incipient 

 crumpling of the folia (PI. X. fig. 7). Some of the lines of black 

 dust are bent back upon themselves in the way so familiar in mica 

 schist and gneiss. Instances also occur where a similar crumpling 

 is presented by the sericite and chlorite. 



There cannot, I think, be any hesitation in affirming that the 

 foliation of these fine schists has had nothing whatever to do with 

 the protrusion of the granite and quartz porphyries. It is not 

 specially developed near these rocks, and, on the other hand, is 

 admirably exhibited at a distance from them. I am inclined to 

 believe that not only is the foliation independent of the eruptive 

 rocks, but it took place long before their protrusion. It was 

 probably connected with plication, as appears to have been so 

 generally the case in areas where rocks have been subject to this 

 kind of metamorphism. The eruptive rocks themselves show no 

 trace of foliation ; but they could hardly have escaped this change 

 had they already been in position when the schistose structure was 

 being superinduced upon the adjoining strata. 



What renders the foliation of the St. David's area so interesting 

 is its feeble development, and its singularly sporadic and almost 

 capricious distribution. In many places one cannot always decide 

 whether to regard a given rock as a true foliated schist or simply 

 as a shale. The same part of a group is shaly at one locality and 

 schistose at another. Some strata seem to have been able to resist 

 the change throughout the district ; the red shales above the con- 

 glomerate, for example, remain true shales, though some of the bands 

 of tuff intercalated in them show faint foliation. Others, again, 

 have been prone to change. This appears to have been particularly 

 the case with some kinds of fine volcanic debris. The microscopic 



