282 A. GEIKIE ON THE SUPPOSED 



lavas, alternating with felsitic breccias and halleflintas"*. In 

 another paper he writes that " they were marked on the Geological 

 Survey Maps as intrusive felstones ; but a very cursory examination 

 proved that they were not of that nature, and that they were in 

 reality bedded sedimentary rocks which had undergone metamorphic 

 change " f . Passing over the discordance between these two emen- 

 dations of the Survey Maps, I would observe that the author, in dis- 

 missing the view taken by the officers of the Survey and substituting 

 for it another of his own, offers no observations of any kind in support 

 of his emendations. He simply declares the rocks to be rhyolitic 

 lavas (meaning evidently, streams of lava that have flowed out at 

 the surface), but mentions no character by which they are to be 

 distinguished from intrusive masses. In a previous paper, indeed, 

 he had admitted that they were " possibly intrusive "$, though at 

 the same time he regarded them as " appearing distinctly to lie in 

 the line of bedding of their associated quartz rocks." He would 

 seem to have been led to regard them finally as lavas, from a remark 

 made to him by Professor Bonney that they most resemble a lava- 

 flow §. I presume it was the presence of fluxion-structure in them 

 that suggested this identification ; but I shall subsequently show how 

 fallacious this presumed test is for the purpose of distinguishing the 

 superficial from the more deep-seated manifestations of volcanic 

 matter. To go no further than the region of St. David's, I find 

 spherulitic structure and fluxion structure in the most obviously 

 intrusive dykes. 



In every example in this district where the actual contact of the 

 porphyries with the surrounding stratified rocks can be seen, the 

 porphyries are distinctly intrusive. In the quarries north of the 

 Church Schools the fine tuffs and schists or shales, which are un- 

 doubtedly a portion of Dr. Hicks's " Pebidian " group, are much 

 indurated close to the porphyry, which traverses them obliquely to 

 their bedding. But this alteration insensibly dies away as the strata 

 are followed northward ; and at a distance of about sixty or seventy 

 yards they assume their usual characters of fine tuff. The actual 

 intrusion of one of the quartz-porphyries as a dyke or elvan 

 through the strata, however, may be seen in the noble section 

 among the cliffs south of Nun's Chapel. Other examples occur 

 further west, near Treginnis. The behaviour and structure of these 

 rocks will be discussed in Part II., in connexion with the metamor- 

 phism of the district. 



Dr. Hicks associates certain breccias with his rhyolites as con- 

 temporaneous components of the " Arvonian group." But there can 

 be no doubt that they are portions of the volcanic (or what he 

 terms his "Pebidian") group which have been invaded by the por- 

 phyries and have been much indurated. They can be seen north of 



* Proc. Geol. Assoc, vol. vii. pt. 1, p. 62 (1881). 

 t Quart. Journ. Geol. Soc. vol. xxxv. p. 286 (1878). 

 $ Quart. Journ. Geol. Soc. vol. xxxiv. p. 153 (1878). 

 § Ibid. p. 154. 



