﻿1851.] MURCHISON SILURIAN ROCKS OF SCOTLAND. 



155 



tured, and distorted aspect, is probably affected by tbe trappean 

 eruption to which Knockdolian Hill and the valley of the Stinchar 

 owe their origin. At Aldeans quarries, about three miles N.E. of 

 Colmonel, and in hills above the right bank of the Assell, the same 

 limestone as that of the Bogang Farm and Craig Neil is quarried in 

 dislocated masses. This dark-coloured rock, containing Maclurea 

 and the same fossils as in the other localities, is in most parts either 

 thick-bedded or amorphous, of a highly altered character, and very 

 fetid under the hammer. As at Bogang, trap and serpentine protrude, 

 and one fine specimen of Maclurea as well as the smaller fossils 

 obtained are coated over with serpentinous matter. Here the rock 

 has been wrenched, contorted, and partially deflected from the true 

 strike ; for at the east end of the quarries the beds farthest removed 

 from the centre of disturbance, and which are laminated and nodular, 

 dip 40° to the E. and by S., or away from the trappean eruptions. 



It is worthy of remark, that parts of the surface of the limestone 

 have been covered with a pebbly conglomerate, which, although it 

 dips generally with the inferior limestone, seemed to me to overlap it 

 with a slight unconformity, or at all events to have been deposited 

 on the eroded surface of the limestone. This point of observation 

 is not to be neglected by those who may hereafter be called upon to 

 decipher more closely the features of this contorted region ; for it 

 might prove the existence of a dislocation in the South of Scotland, 

 which may explain the occurrence of the very coarse conglomerates, 

 and also lead to the identification of some of the sandstones with the 

 true Caradoc, and of others with the sandstones and pebble-beds of 

 Llandeilo age, delineated by Prof. Ramsay as lying unconformably 

 beneath the Caradoc*. It occurred to me, therefore, that if the con- 

 glomerate of Kennedy's Pass be one of the highest in the district, 

 this of Aldeans might be one of the lowest or oldest ; since the fossils 

 of these black limestones induce us to consider them as belonging to 

 the most ancient recognizable Silurian band of this region. It is 

 indeed manifest, that the whole of the series of the shelly sandstones 

 of this district is diversified and swollen out by courses of conglome- 

 rate, which, however thick and powerful in one locality, thin out 

 upon the strike and are not persistent in the general masses of the 

 greywacke of the South of Scotland. The Silurian conglomerates of 

 Scotland, therefore, are to be viewed as phaenomena of much more 

 local character than those of the Old Red Sandstone. 



In regard to the chief limestones, I believe that they are all of the 

 same geological age, and that, together with the schists associated with, 

 and beneath them, they form the lowest band of the Lower Silurian 

 group of Ayrshire f. The species of Orthidce and the Trilobites they 



* See Journ. Geol. Soc. vol. iv. p. 296. 



f The presence of a form of Maclurea would not in itself be sufficient evidence, 

 the genus having been found in rocks of Devonian age at the Rittberg, two Ger- 

 man miles S.W. of Olmutz in Moravia, by M. de Verneuil, Count Keyserling, and 

 myself. We there found it with an abundance of Devonian fossils in deep-coloured 

 limestones overlying other rocks more or less metamorphosed which are probably 

 of Silurian age (see Jameson's Phil. Journ. Edin. 1847, vol. xliv. p. 77). 



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