176 STKATIGKAPHICAL GEOLOGY 



up into thin-bedded flags. This group is more than 2000 feet in 

 thickness. Fossils are abundant, the following being some of the 

 most characteristic : Lingula cornea, Discinci rugata, Spirifera 

 elevata, Grammysia cingulata, Orthoceras ludense, and Trochoceras 

 ibex. 



4. SoutJi, of Scotland 



The Silurian succession in this region was first deciphered by 

 Professor Lapworth, 1 " who described the two special facies exhibited 

 by the rocks of Valentian age near Moft'at in Dumfries, and near 

 Girvan in Ayrshire. More recently the whole region of the 

 southern uplands has been described in detail by Messrs. Peach 

 and Home. 18 The Silurian rocks occupy the southern half of the 

 upland region in a broad belt from 20 to 30 miles in width, 

 extending from the Berwick coast on the east to that of Wigtown 

 on the west. On the northern side they occur as isolated exposure 

 in Ayrshire, in Lanark, and in the Pentland Hills. 



Valentian. In Dumfries and the central belt the lowest beds 

 are soft black shales, known as the Birkhill shales ; they lie 

 conformably upon the Hartfell shales (Bala), are equivalent to the 

 Skelgill shales of Westmoreland, and are of about the same thickness 

 (140 feet) (see Figs. 40 and 41, p. 133). They are rich in grapto- 

 lites, and are divisible into the following zones : 



Zone of Rastrites maxinms. 



Monograptus spinigerus. 

 Petalograptus cometa. 

 Monograptus gregariu.s. 

 Diplograptus venculosus. 

 Diplograptus acuniinattis. 



Lower I 

 Birkhiin 



These shales are succeeded by a thick series of sediments, from 

 3000 to 4000 feet, which are the equivalents of the Browgill 

 Beds and the Tarannon shales. They are called the Gala Group by 

 Professor Lapworth, and have been divided by the Survey into three 

 stages, but fossils are rare except in the lowest or Abbotsford flags, 

 which contain Monograptus turriculatus, M. cnspus, M. exigiius, etc. 



In the Girvan area the Birkhill shale is represented by a much 

 thicker series of a Llandovery type, and about 1000 feet thick. 

 The lower beds contain Lower Llandovery fossils, while the suc- 

 ceeding beds include a limestone with many Upper Llandovery 

 fossils, such as Pentamerus oblongus, Stricklandia lens, Illcenus 

 cemulus, and Phacops Stokesi. Above this, and representing the 

 Gala Group, are the Penkill Beds (also 1000 feet thick), the 

 topmost of these being mudstones containing Cyrtograptus Grayw. 



