﻿1851.] MURCHISON — SILURIAN ROCKS OF SCOTLAND. 145 



lations, exposes masses of highly ferruginous and reddish sandstone, 

 in which we detected portions of the Calymene Blumenbachii or 

 " Dudley fossil" of old writers. 



This fossil becomes, indeed, abundant in certain schists or mud- 

 stones (1) which seemed to me to overlie the mass of the shelly sand- 

 stone and conglomerates. These schists occupy the low slopes of a de- 

 pression to the east of the farm of Drummuck, and at all events form 

 a flanking part of the same range as the sandstones. As the adjacent 

 shelly sandstones undulate, they appeared to me to trough this Trilo- 

 bite-shale. Seeing that they contain the true Calymene Blumenbachii 

 (as identified by Barrande and other naturalists) as well as the Cheiru- 

 rus (Paradoxides) bimucronatus (Sil. Syst.), I am, I confess, disposed 

 to think, that if pertaining to the Lower Silurian division, these soft 

 schists (1) lie in the upper zone of its Scottish development. I know, 

 indeed, that these species of Trilobites are no longer exact chronome- 

 ters of the age of the strata, and are almost universally ' Silurian/ one 

 of them being found from the Ludlow rocks even down, as I am now 

 assured, to the slates of Snowdon ! But here they are unquestionably 

 associated with rocks which, although chiefly of the Lower Silurian 

 age, show signs in their fossils of an ascending order. When followed 

 in their strike to the W.S.W. or to the sea-shore, these strata of shelly 

 sandstone are entirely denuded, and are unconformably overlaid by the 

 carboniferous? red sandstone above spoken of, which forms ledges on 

 the beach (c of Map). 



A great mass of limestone at Craig Head, on the N. bank of the 

 Girvan Water, still remains to be assigned to its geological place (see 

 Map) . The chief difficulty in doing so arises from the fact, that large 

 masses of amorphous greenstone and amygdaloidal trap flank it on 

 the S.W. and N., and thus cut it off from the strata above described. 

 No Silurian strata are there visible, towards the carboniferous valley 

 of the Girvan, or eastwards ; whilst the signs of bedding in the lime- 

 stone are almost obliterated. 



This limestone is laid open by quarries from 60 to 70 feet deep, 

 and occupies the lower portion of undulating hills which consist, in 

 this higher part, of trappean amygdaloid and greenstone. The rock 

 is of a grey colour, chiefly amorphous, with irregularly intercalated 

 portions of greenish earth. The limestone is white-veined, and pre- 

 sents, even at first sight, the aspect of being of Silurian age. Although 

 I only detected imperfect traces of fossils, Mr. J. Carrick Moore has 

 obtained quite a sufficient number, not till now announced, to lead us 

 to affirm that the rock is unquestionably Lower Silurian, and of the 

 same age as other limestones hereafter to be described. These fossils 

 having been examined by Mr. Salter, prove to be — 



Orthis elegantula, Balm. 



Orthis confinis, Salter, PI. VIII. fig. 5. 



Orthis, n. sp. 



Leptsena or Strophomena, 1 or 2 species. 

 Terebratula, an obscure species, PI. VIII. fig. 3. 

 Pleurorhynchus dipterus, Salter, PI. VIII. fig. 6, 



