XXviii. METEOROLOGY — GEOLOGY OF DUMFRIESSHIRE. 



table-land that stretches from St. Abb's Head to Portpatrick. There is 

 little variation in the lithological characters of the rocks, as they consist 

 mainly of massive grits, greyvvackes, flags and shales singularly destitute 

 of fossils. Fortunately, however, there are certain bands of black shales 

 richly charged with graptolites, by means of which Professor Lapworth 

 demonstrated the true order of succession of the beds. The strata 

 have been thrown into innumerable folds, frequently inverted, whereby 

 certain zones, not exceeding several hundred feet in thickness, have 

 been made to cover areas several miles in width. Further, the members 

 of the black shale series, which are typically developed in the Moffat 

 region, undergo important modifications when followed north-westwards 

 to Wanlockhead. The higher fossiliferous zones gradually disappear, 

 and they are represented by coarser sediments. For these reasons the 

 stratigraphical relations of the strata are extremely complicated, but 

 with the aid of the graptolites it is possible to determine the age of the 

 various rock groups, and to correlate them with the subdivisions of the 

 Silurian system in other regions. 



The lowest zones of the Moffat black shale series are accompanied 

 by cherts containing Radiolaria which were detected in the course of 

 the geological survey of the Abington area. The latter form an im- 

 portant horizon from their great horizontal extension and their constant 

 association with volcanic rocks. From these cherts Dr Hinde has 

 described twenty-three new species of Radiolaria belonging to twelve 

 genera, of which half are new. He concludes that these Silurian cherts 

 from the south of Scotland are due to the accumulation of the tests of 

 Radiolaria, forming a pure Radiolarian rock resembling the Teritary 

 beds of Barbados and the Nicobar Islands, which, according to Hgeckel, 

 correspond to the Radiolarian ooze of existing seas. There can be no 

 doubt, therefore, that these cherts are true deep-sea deposits, which 

 must have accumulated beyond the limit of sedimentation. The horizon 

 of this important zone is accurately defined in the Ballantrae area, Ayr- 

 shire, where the cherts are underlain by a band of black shales contain- 

 ing Avenig graptolites. Throughout the southern uplands they are 

 always overlain by the Glenkiln black shales yielding graptolites of 

 Upper Llandeilo age. Hence they belong partly to Avenig and partly 

 to Llandeilo time. 



The following table gives in descending order the subdivisions 

 of the black shale series, which have been established by Professor 

 Lapworth in the typical Moffat region : — 



f Upper Birkhill TZone of Rastrites maximus, Carr. 

 I or < Zone of Monograptus spinigerus, Nich. 



fjrey Shale Group. [Zone of Cephalograptiis comela, Gein. 



Llandovery.. 



Lower Birkhill ("Zone of Monograptus gregarius, Lapw. 

 or < Zone of Diplograptus vesiculosus, Nich. 



, Black Shale Group. ( Zone of Diplograptus acuniinatus, Nich. 



