GEOLOGY OF DUMFRIESSHIRE. 



XXIX. 



Upper Hartfell f Zone of Dicellograptus anceps, Nich. 



or 

 Barren Mudstones. I Zone of Barren Mudstones. 



CARADOC 



LLANDEILO . 



Lower Hartfell 



or 

 Black Shale. 



Glenkiln 

 Black Shales. 



Avenig. 



( Zone of Pleurograptus linearis, Carr. 

 -| Zone of Dicranograptus Clingani, Carr. 

 [ Zone of Climacograptus Wilsoni, Lapw. 



[ Comograptus gracilis, Hall. 

 -! Thamnograptus typus, Hall. 

 [ Didymograptus superstes, Lapw. 



I Radiolarian cherts, mudstones, and volcanic 

 I tuffs. 



The typical sections, where the various divisions of the black shale 

 series are displayed, occur within the limits of the county, at the locali- 

 ties from which they take their name. The members of the lowest 

 division, corresponding to the Upper Llandeilo rocks of Wales, are met 

 with in the Glenkiln burn, near Raehills, to the south-west of Moffat ; 

 those of the middle division, representing the Caradoc rocks, occur on 

 Hartfell at the spa, north-west of Moffat; while the members of the 

 Upper division of Llandovery age, are seen to advantage in the famous 

 section in Dobb's Linn, near Birkhill, at the head of Moffatdale. 



The representatives of the black shale series, in the Moffat region, 

 are exposed along axial folds in the midst of younger strata. They are 

 arranged in parallel bands running in a north-east and south west direc- 

 tion, which can be readily distinguished from the surrounding grey- 

 wackes and shales by their colour and composition. Where the folds 

 are normal, the lowest beds occur in the centre, and the higher zones 

 follow in regular order on either side. Frequently the arches are in- 

 verted, and both limbs dip in one direction, while the beds are traversed 

 by normal and reversed faults. In order to study the succession of the 

 strata with advantage, the observer must visit the typical sections of the 

 Moffat region, where the various lithological and palaeontological zones 

 are admirably displayed. 



One of the remarkable features of these Silurian rocks, ranging from 

 the horizon of the Radiolarian cherts to the top of the Birkhill shales, 

 is the variation in the character of the deposits when followed across 

 the strike of the beds, that is from south-east to north-west. This vari- 

 ation, together with the complicated system of folding, has led to much 

 of the difficulty in interpreting the succession. Excluding the Radio- 

 larian cherts, which are evidently true oceanic deposits, the strata in the 

 Moffat region consist of black and grey shales, clays and mudstones in- 

 dicating deposition in comparatively deep water. The whole series 

 does not exceed three hundred feet in thickness, and yet on palaeonto- 

 logical grounds it is evident that the beds represent the Llandeilo, Cara- 

 doc, and part of the Llandovery formations of Wales, which in the latter 



