﻿1851.] MURCHISON SILURIAN ROCKS OF SCOTLAND. 159 



cudbright belong to one and the same group, which is, on the whole, 

 a younger formation than the Ayrshire rocks, and must, I conceive, 

 be classed as Upper Silurian. 



C. Dumfriesshire ; Sections across the Central Silurian 



Region. 



I will be as brief as possible in stating the principal facts which 

 fell under the notice of my associate and self in the very general sur- 

 vey we made of Dumfriesshire. This country had previously been 

 examined by Professor Sedgwick, who had collected many Grap- 

 tolites from it, which have been recently described by Prof. M'Coy. 

 Mr. Harkness had since our visit contributed many details* concern- 

 ing what he has termed the Silurian rocks of that district, and has 

 also described some Graptolitesf. In examining these rocks, even 

 up to the extreme northern frontier, near Sanquhar, there is an 

 almost total absence of limestone and of any beds lithologically re- 

 sembling the shelly sandstones of Ayrshire. The Caledonian railroad 

 has, in fact, laid open an excellent section of all the strata from Lock- 

 erby on the south by Beattock near Moffat. Attaining its summit- 

 level in the wild hills whence the sources of the Clyde, the Evan, the 

 Annan, and the Tweed arise, this railroad descends to Abington be- 

 fore it emerges from the rocks of this age and enters into the coal- 

 fields of Lanarkshire. 



In examining the cuttings near Beattock, Prof. Nicol and myself 

 detected one anticlinal north of that place, at which hard purple 

 grey wacke-sandstone passes under black carbonaceous and anthracitic 

 schists, which in the bed of the rapid burn and deep glen of Garpool 

 has afforded Graptolites to the persevering search of Mr. Harkness. 



We also recognised the continuation of the great trap-dyke J of 

 Coates Hill above Moffat, which, striking from N.W. to S.E., re- 

 appears at Craig Fell Hill on the right bank of the Annan, and is 

 probably continuous to the igneous dykes on the Esk near Lang- 

 holme, whose embranchments pass near Dumfries §. 



Besides the flexure near Beattock, we observed, between that place 

 and Aachen Castle, that beds, consisting of strong greywacke-flagstones 

 or schists, often nearly vertical, dip at high angles to the S.S.E., whilst 

 on the ascent of the railroad, as seen at the Greskin cuttings, and at 

 other places, they plunge sharply to the N.N.W. at an angle of 75°. 

 This latter dip is apparently continuous for some miles, the strata 



* Quart. Journ. Geol. Soc. vol. vii. p. 46. 



f Ibid. vol. vii. p. 58. & PI. I. % hoc. tit. p. 48. 



§ No geologist has previously described this very remarkable dyke ; still less has 

 it been laid down in a map, though its length probably exceeds twenty-five miles. 

 At Coates Hill it is an amygdaloidal basalt ?, the fine prisms of which dip 75° N.W., 

 their ends forming a peculiar mass in contact with the greywacke which they 

 traverse. On the faces of the joints some serpentine appears. The kernels in the 

 mass consist of white carbonate of lime, smaller portions of which are so diffused 

 through the rock with chlorite and minute crystals of Labrador, that, like the 

 prismatic trap of Welshpool, it forms an excellent and tractable building stone 

 (see ' Silurian System/ p. 288). 



