134 STKATIGEAPHICAL GEOLOGY 



graptolites were characteristic of Llandilo Beds and did not occur 

 outside their limits. The true structure and succession of strata 

 in this region was first established by Professor Lapworth, whose 

 researches on the Girvan and Moffat districts are embodied in two 

 detailed papers, and who afterwards summed up the structure of 

 the whole region in another one, to which the student should 

 refer. 16 More detailed descriptions are given in the " Geology of 

 the Southern Uplands," Mem. Geol. Surv. 1899. The sections, 

 Figs. 40 and 41, are taken from Lapworth's papers. 



The special interest of this region is that it presents us with 

 two very different facies of sedimentation, which, however, can be 

 traced through changing intermediate facies from the one area to 

 the other. In Ayrshire (Girvan, etc.) the Ordovician has the 

 ordinary facies of a formation accumulated at no great distance from 

 a continental coast-line, consisting as it does of a considerable 

 thickness of conglomerates, sandstones, shales, with at least one bed 

 of limestone. The other type, found in Dumfries and Wigtownshire, 

 is one that must have been formed in much deeper water, for there 

 the whole system is condensed into a small thickness of dark- 

 coloured muds tones, shales, and chert - beds. The base of it 

 is not exposed, but the total thickness from the summit of the 

 Bala Series down to the base of the Arenig chert-beds is only about 

 185 feet, as compared with a thickness of 3500 feet in Girvan. 



Professor Lapworth divided the Ordovician of this region into 

 three groups or series, which are roughly equivalent to Arenig, 

 Llandilo, and Bala, and these divisions have been adopted by the 

 Geological Survey of Scotland. The graptolitic facies of the 

 formation lends itself to easy zonal subdivision and to correlation 

 with other areas, so that the names given to the middle and upper 

 parts of this succession, i.e. Glenkiln shales and Hartfelt shales, 

 have been widely used as if they were synonymoxis with Llandilo 

 and Bala. The progress of discovery in Wales, however, has shown 

 that the divisional line between the Glenkiln and Hartfell shale 

 is not quite on the same horizon as that taken in South Wales 

 between the Llandilo and Bala. 



Moreover, nothing comparable to the Llanvirn Series has yet 

 been identified in Scotland. No shales yielding Didymoyraptus 

 bifidus or Did. Murchisoni have been found. In the one area there 

 is a distinct unconformity at the base of the Llandilo with a 

 conglomerate above the break ; in the other there is merely an 

 abrupt change from one kind of sediment to another. It is evident, 

 however, that a chapter of the geological record is missing in this 

 region, and the following tabular view gives the established divi- 

 sions without any attempt to correlate them with those of Wales. 



