CHAP. III.] ORDOVICIAN PERIOD. 33 



they are so entangled among igneous and metamorphic 

 rocks that their stratigraphical relations have not yet been 

 made out. Moreover, it appears certain that there is a 

 marked unconformity between the igneous group, or Bal- 

 lantrae rocks, and the succeeding deposits of Upper Llan- 

 deilo age. At the base of the latter is a coarse purple 

 conglomerate containing fragments of the Ballantrae rocks, 

 and we may therefore conclude that land existed in this 

 area during the greater part of the Arenig and Llandeilo 

 periods. Moreover, the base of the Bala series is a con- 

 glomerate 500 feet thick, which overlaps the Llandeilos on 

 to the older rocks, so that some land was then still above 

 the sea-level, but this was soon submerged, and the subsi- 

 dence continued throughout the Bala period till a thick- 

 ness of 3,300 feet of deposits had been laid down. 



That the land thus indicated did not extend far in a due 

 easterly direction seems probable from a consideration of 

 the Moffat section ; here no base is exposed, but the small 

 thickness and uniform shaly character of the deposits, 

 taken together with the rarity of any organic remains ex- 

 cept graptolites, lead us to infer that these shales were 

 formed at some distance from land and in comparatively 

 still and deep water. It is possible that the land may 

 have had a south-westerly prolongation into the north of 

 Ireland, but the base of the Ordovician is not there seen, 

 and consequently there is no actual evidence for this sup- 

 position. 



With regard to the northern part of Scotland, it is only 

 quite recently that its structure has been properly under- 

 stood. Messrs. Lapworth and Callaway were the first to 

 establish the true upward succession, and to show that it 

 was in many places inverted by the complete overthrow of 

 the strata. Their work has been continued by the officers 

 of the Geological Survey, and the succession in Durness 

 and Eriboll is given as follows, in descending order : 



D 



