310 A. GEIKIE ON THE SUPPOSED 



nant strike of the stratified rocks without strictly conforming to it. 

 I have shown how the granite cuts into successive zones of the 

 Cambrian series, until it invades the group of greenish sandstones 

 and shales lying above the conglomerate. The most serious distur- 

 bance of the regularity of the beds on the south-eastern side of the 

 fold occurs in the Allan valley between Ehoscribed and Porth- 

 clais. There appears to be a minor plication between the two 

 projections of granite, as expressed on fig. 1, p. 268. Beyond this 

 local disturbance the strata can be followed eastward until the re- 

 versed dip of the isocline changes to vertical ; and the beds rapidly 

 lessen in inclination as group after group of the Lower Cambrian 

 series appears in normal order. 



The St. David's area is one of the numerous tracts of Pembroke- 

 shire where eruptive masses have risen in a general north-east 

 and south-west direction parallel with the principal stratified rocks. 

 Round the granite are grouped various quartz porphyries; and 

 similar rocks probably continue to rise in detached bosses or dykes 

 along the same line of extravasation towards the north-east. 



From the way in which the granite and porphyries traverse the 

 stratified rocks, it may be confidently inferred that they are of later 

 date than the general plication. At the same time there is evidence 

 of movement subsequent to the intrusion of these rocks ; for slicken- 

 surfaces may often be noticed on their joints. 



In the first part of this paper I have admitted that minor local 

 displacements may have occurred here and there along the edge of 

 the granite ; but they do not in any way affect the general geological 

 structure of the district. That no large or important faults occur 

 is made quite certain by the remarkably clear coast-section, and by 

 the evidence of the transverse valley of the Allan river, in which, 

 in spite of the intrusion of the granite, the normal succession of beds 

 can be distinctly recognized. If, indeed, the granite could be elimi- 

 nated from the district, the various groups of strata on either side of 

 it would close up into their usual order. 



The latest rocks are the dykes of diabase, by which all the others 

 are traversed. These show occasionally a kind of cleavage or fine- 

 jointing, which may perhaps point to renewed lateral pressure after 

 their extrusion. 



Cleavage was not observed to characterize the stratified rocks, 

 though we noticed a few feeble examples of it. But the fine folia- 

 tion already referred to is conspicuous in the volcanic group and on 

 several horizons in the groups overlying the conglomerate. To this 

 structure I shall devote a separate section of the paper. 



3. The Foliation of the Disteict. 



A fine foliation, arising from the development of micaceous mine- 

 rals along the planes of stratification, has been extensively developed 

 in the volcanic group, and likewise in the groups of ordinary sedi- 

 mentary strata overlying the conglomerate. It has affected many 

 of the fine tuffs, the paste of certain coarser tuffs, and some of the 



