302 A. GEIKIE ON THE SUPPOSED 



may be observed also in detached nodules and in strings and veins 

 crossing the bedding of the strata. 



To this substance various names might be given, according to the 

 varying circumstances under which it is found. Much of it might 

 be classed with the " siliceous schist " or " Kieselschiefer " of the 

 older petrographers ; some of it assumes the characters of the " Horn- 

 schiefer " found in areas of contact-metamorphism. In other places 

 it resembles the eurites or halleflintas of regional metamorphic 

 areas. Occasionally it becomes almost sufficiently flinty and trans- 

 lucent to deserve the name of chert. Prom the analysis kindly 

 made of it for me by H. Renard, some portions answer exactly to 

 Beudant's " adinole " — a term which has been revived by German 

 petrographers. It is obviously not a definite chemical compound, 

 nor has it any uniform microscopic structure. It includes the 

 " halleflintas " of Br. Hicks. 



To what extent the silica of these aggregations is due to ori- 

 ginal deposition, is a problem to which I shall recur in the sequel. 

 From the fact that the cherty material ramifies in veins across the 

 bedding, its introduction must certainly, to some extent at least, 

 be later than the deposition of the shales and tuffs which it traverses. 

 I shall be able to show, indeed, when describing it more fully in a 

 later part of this paper, that its appearance has certainly been, in 

 some cases, later than that of the quartz porphyries, and that its 

 production has been connected with the general process of extru- 

 sion of the highly silicated rocks of the granite tract. 



Lavas of the Volcanic Group. — There remain for notice here the 

 sheets of eruptive rocks that occur among the tuffs. Excluding the 

 granites and porphyries (to which a special section of this paper will 

 be devoted), two kinds of eruptive rocks are associated with the 

 volcanic zone. One of these is certainly intrusive and of late date, 

 viz. dykes and veins of diabase, which will be described in later 

 pages. The other kind occurs in long parallel sheets, some of 

 which, if not all, are true contemporaneous lava-streams, erupted at 

 intervals during the accumulation of the volcanic group. They 

 form prominent crags to the west of St. David's, such as Clegyr 

 Foig, Ehosson, and the rocky ground rising from the eastern shores 

 of Ramsey Sound. Their dip and strike coincide with those of the 

 tuffs above and below them. It is possible that some of these 

 sheets may be intrusive along the bedding of the tuffs ; and in one 

 or two cases I observed indications of what, on further and more 

 careful exploration, may prove to be disruption across the bedding. 



But it is the interbedded sheets that possess the chief interest as 

 superficial lava-streams of such venerable antiquity. They present 

 many of the ordinary features of true lava-flows. In particular a 

 slaggy structure may be detected at the bottom of a sheet, the 

 vesicles being here and there lengthened in the direction of flow. 

 Some of the sheets are in part amygdaloidal. The alternation of 

 these sheets with tuffs, evidently derived from lavas of similar cha- 

 racter, is another argument in favour of their contemporaneous 

 date. One of the best localities for studying these features lies 



