318 A. GEIKIE ON THE SUPPOSED 



no distinct metamorphism at all. But even a large mass may pro- 

 duce little alteration. There seems to be some relation between 

 the mineral constitution of the granite and the nature and amount 

 of the metamorphism which it may superinduce *. 



The importance of noting these two conditions of the problem is 

 well shown at St. David's. The granite mass there is but of small 

 dimensions, and it is where it narrows into minor projections that 

 its contact with the adjacent rock is chiefly exposed. These are 

 precisely the circumstances under which only a feeble degree of 

 metamorphism might be expected. On the other hand, where the 

 rocks next the granite have not been much decomposed, they are 

 found to be quartz conglomerate, or quartzose grit, in which little 

 or no trace of alteration need be looked for. 



The best natural section for noting the alteration produced by the 

 granite is that exposed on the cliff at Ogof-llesugn. The conglo- 

 merate has there been indurated into the consistency of quartzite, 

 breaking readily across the pebbles. The grits in the Allan valley 

 have been similarly affected, but in a less degree. Microscopic 

 preparations of these rocks show a structure like that of the 

 quartzites of the Highlands. The quartz grains and pebbles have 

 suffered no apparent change, except that in some places they have 

 been much fractured. A deposit of secondary quartz may be observed 

 running in veins through the rock. Where the pebbles have been 

 fractured, disrupted portions are imbedded in a matrix in which, 

 besides crystalline quartz, an indistinctly fibrous substance occurs, 

 which may be chalcedony. 



The shales and fine grits near the granite at Porth-clais are beau- 

 tifully foliated, lines of the bright-green chloritic ingredient already 

 referred to being especially prominent between the bands of quartz- 

 grains. But, for reasons already given, I believe that this foliation 

 has not resulted from the influence of the granite. At the same time 

 there may be room for inquiry whether the effect of the granite may 

 not have been to set up a new foliation, which, where it coincided in 

 direction with the first, might intensify it. 



From a specimen of fine shale or schist taken from near the 

 granite on the right bank of the river, I have had several slices 

 prepared. In these the first foliation is excellently shown ; but the 

 folia have been ruptured and shifted by, as it were, a series of closely 

 parallel faults, along which a new but more feeble foliation has been 

 developed by the production of a fine white mica (Plate X. fig. 8). 



The alteration round the granite appears not to extend many 

 yards away from the eruptive rock : but the sections are few and 

 limited in • extent ; and in some places, owing to small local slips, 

 the strata now abutting on the granite may have been originally at 

 some little distance from it. In the case of the porphyries, however, 

 the sections are far more numerous and extensive. The metamor- 

 phism associated with these rocks is also more marked. It consists 



* Professor Zirkel informs me that in his experience granites with white mica 

 alter the surrounding rocks little, or not at all, and that it is the granites with 

 black mica that produce most of the metamorphism. 



