264 A. GEIKIE ON THE SUPPOSED 



A second (revised) edition of the Map was published in 1857, and 

 was soon followed by a second edition of the Sections. The primary 

 object in the revision of the work was the tracing of a line for the 

 base of the Lingula-flags ; but the opportunity was also used for 

 separating some of the more important varieties among the igneous 

 rocks. These were now classed as " syenite " and " greenstone," 

 all mention of the " volcanic ash " of the previous edition being 

 omitted from the St. David's area. A considerable tract to the west 

 of St. David's, which had been mapped by Ramsay as igneous rock, 

 and which was regarded by him as composed mainly of volcanic ash, 

 was now coloured as " altered Cambrian"*. 



As thus revised, the Map showed a long strip of syenite and f el- 

 stone, stretching from the sea through St. David's in a north-easterly 

 direction for about seven miles, flanked on the south-east by Cam- 

 brian strata, and on the north-west by " altered Cambrian " rocks 

 through which long parallel sheets of greenstone had been erupted. 

 The existence of abundant contemporaneous igneous rocks further 

 north and east is clearly shown on the map ; but in the immediate 



drawn up by him at the time, but never published, has fortunately been also 

 preserved. From this interesting document a few extracts may here be appro- 

 priate : — "The igneous rocks of North Pembrokeshire are both intrusive and con- 

 temporaneous ; the latter, however, occupy by far the greater area. The greatest 

 intrusive mass is that which from Ramsay Sound stretches in a north-easterly 

 direction nearly eleven miles. The best evidence of this trap being intrusive 

 may be seen on the coast near St. David's, at Ogof-llesugn, where it cuts through 

 the strata... at right angles to the line of strike. 



"The apparent composition of this large mass is very various, resulting, no 

 doubt, in many instances from the different circumstances under which it cooled 

 and consolidated. Thus, on the east side of Porth Lisky, and on both sides of 

 the Allan at St. David's, it is a large-grained, coarsely crystallized syenite, 

 hornblende being comparatively sparingly diffused throughout. At Porth Lisky 

 granular crystals of quartz are largely developed. In other places the same 

 development may be seen. 



" The coast of Porth Lisky is almost entirely composed of volcanic ash, which, 

 being of a softer texture than the tx*aps on the E. and W. sides of the bay, has 

 yielded to the action of the waves, and thus formed a little harbour. ... From 

 hence, in the direction of Ramsay Sound, the rocks are composed of hard sili- 

 ceous trap [and] various greenstones. ... These frequently alternate with partial 

 layers of volcanic ash. These varieties constitute the rest of the mass to the 

 eastward, without any very apparent order in the manner of their distribution. 

 ...NearTrelethin (St. David's) and elsewhere there is a little volcanic conglo- 

 merate. With the exception already mentioned, the strike of the strata generally 

 more or less conforms to the general run of the intrusive mass. 



" A glance at the contemporaneous traps shows that volcanic agencies had 



been in operation for long successive periods From the intermixture of 



volcanic ash and greenstone at Porth Lisky, it would appear that the forma- 

 tion of this mass was in some measure the work of successive eruptions." 



* Professor Ramsay, as Local Director of the Survey, agreed to this change, 

 which was made on the ground by Mr. Aveline. In many respects the map was 

 a marked improvement upon the first edition ; but the suppression of the refer- 

 ence to fragmental volcanic rocks and the introduction of the term " altered 

 Cambrian " were unfortunate changes, one effect of which has been to obscure* 

 the fact that to Ramsay belongs the merit of having first clearly recognized the 

 presence of truly contemporaneous fragmental volcanic rocks in a formation of 

 such high antiquity. 



