PRE-CAMBRIAN ROCKS OF ST. DAVID'S. 317 



way along the bedding-planes of the shales, bnt has also broken 

 across them, sending out finger-like branches. At first I took 

 the rock for a tuff ; and it was not until I noticed that it was 

 porphyritic with quartz, and that it intersected the shales, that I 

 recognized its true character. Some portions are veined with, and 

 contain lenticular seams of the siliceous substance to be immediately 

 described, in which may be detected the doubly terminated quartz 

 and the felspar crystals of the rock. 



The association of quartz porphyry with granite is so familiar a 

 fact as to need no further comment here. I would only add that in 

 the granite district of Criffel and Galw.ay there are masses of quartz 

 porphyry presenting the closest resemblance to those of St. David's. 

 Like the granite with which they are connected, they have risen 

 through Lower Silurian strata. 



I now come to the metamorphism that has attended the intrusion 

 of the granite and porphyries. The district of St. David's is too 

 limited in extent to furnish data for a full discussion of this subject. 

 The facts there attainable ought to be extended by observations of 

 the line of junction of the eruptive and sedimentary rocks in the 

 rest of Pembrokeshire. I would therefore at present offer only such 

 a slight sketch as the material in my possession seems to warrant. 



The metamorphism traceable near the granite and quartz por- 

 phyries appears to consist partly in induration due to the introduc- 

 tion of new mineral matter, notably silica, into the strata, partly in 

 the development of crystals or a crystalline rearrangement of the 

 materials of the adjacent rocks. 



In dealing with the amount of change superinduced upon stratified 

 masses by granite which has been intruded into them, we have two 

 factors in the question to consider — the petrographical structure and 

 composition of the rocks affected, and the character and particularly 

 the bulk of the eruptive mass. In regard to the first of these two 

 points I may remark that quartz and quartzose rocks present little 

 scope for metamorphic action. Secondary quartz may be deposited 

 in their fissures, or between their particles ; but unless they contain 

 some silicate or other mineral matter which may be susceptible of 

 recomposition and recrystallization, they may show no further change 

 than mere induration from introduced silica, or from the solution 

 and recementing of their component grains. 



With respect to the part played by the granite, it must be remem- 

 bered that marked metamorphism does not always accompany intru- 

 sions of this rock*. If the mass of granite be small, there may be 



* In this country examples may be found where little or no alteration is 

 perceptible round the margin of the granite that has undoubtedly been erupted 

 through the adjoining strata. Round the smaller granite bosses of Galloway 

 this is observable among the more quartzose greywackes. The granite of Arran, 

 though so large a mass, only slightly affects the surrounding rocks. On thp con- 

 tinent numerous instances have been observed where no contact-metamorphism 

 occurs round truly eruptive granite. My friend M. Renard has kindly supplied 

 me with the following illustrations: — Petschau in Bohemia, where the contact 

 of granite and shale is as sharp as if cut with a knife ; Greifenheim ; lie de 

 Michau (Cotes du Nord) ; banks of the Irtisch. 



