THE TRILOBITES OF THE SILURIAN ROCKS OF 

 SOUTHWEST SCOTLAND. 



BY B. N. PEACH, F.R.S. 



THE area under consideration is one in which there is an almost unbroken 

 succession of rocks recording continuous deposition from Lower Arenig up to 

 the close of Downtonian time in fact nearly the whole of the Silurian 

 period. During this prodigious lapse of time trilobites became the most 

 prominent marine arthropods, the} 7 culminated in the Caradoc period, waned 

 during Upper Silurian and Devonian time, while their last lingering remains 

 finally disappeared within the Carboniferous period. As will be seen in the 

 sequel the trilobite remains found in the Silurian rooks of the area, and more 

 especially of the Girvan region, strongly bear out this conclusion. 



LOWER SILURIAN. 



During the deposition of the Arenig and Lower Llandeilo rocks of the area, 

 which appear to have accumulated in an open sea free from land-borne sedi- 

 ment during pauses of volcanic activity, the conditions appear to have been 

 so unfavourable to trilobite life that not a vestige of their remains has been 

 recorded from rocks of these horizons. Towards the close of Upper Llandeilo 

 time, however, when earth-movements had reared these pelagic strata above 

 the surface over a great part of the area, trilobites appeared in profusion 

 among the denizens of the shore, and their remains are numerous and well- 

 preserved in the rocks representing the finer sediments derived from the 

 newly emerged land. In the Girvan district the chief horizons in which 

 they occur, -beginning at the lowest strata and working upwards, are as 

 follows : 



The Stinchar limestone, proved by graptolite evidence to be near the top 

 of the Upper Llandeilo rocks, occurs between the massive Kirkland and 

 Benan conglomerates, and, together with its associated mudstones probably 

 representing a pause in the uplift, has yielded 18 species belonging to 

 14 genera, the forms being of Caradoc facies as shown by R. Etheridge, Jnr. ' 



The next horizon is that of the Balclatchie mudstones overlying the great 

 Benan conglomerate. From the graptolites yielded by the associated beds, 

 which are those of the intermediate zone between the Glen kiln and Hartfell 

 subdivisions, these beds must lie somewhere near the base of the Caradoc 

 rocks of the region. These fine green mudstones have been looked upon by 

 some observers as volcanic tuffs, but, like the Benan conglomerate itself, they 

 are evidently only the ordinary detritus from the rocks of the ridged up 

 Arenig volcanic group. They are exposed at Dow Hill and Ardmillan Braes 



i "A Monograph of the Silurian Fossils of the Girvan District in Ayrshire," vol. i. 

 By H. A. Nicholson and R. Etheridge, Jur., 1880. 



