428 THE SILURIAN ROCKS 



Maclurea macromphala, M'-Coy. Conularia sp. 



Macrochilina, sp. Hyolithes, sp. 



Murchisonia angulata, Sow. Pterotheca corrugata, Salt. 



M. gyrogonia, M'Cot/. Cyrtoceras scoticum, Blake. 



Pleurotomaria alata, Wahl. Endoceras, sp. 



PL (Raphistoma) elliptica, His. Lituites, sp. 



PI. (R.) lenticularis, Sow. Orthoceras arcuoliratum, Hall. 



Rhaphistoma sp. 0. Avelini, Salt. 



Turbo, sp. 0. pendens, Blake. 



Conularia elongata, Portl. 



GIRVAN DISTRICT. 



This district embraces an area in the extreme northwest part of the table- 

 land, extending along the shore from Girvan to the mouth of Glen App, 

 and inland to the village of Barr in the valley of the Stinchar. 



The geological structure of the area south of the Stinchar closely resembles 

 that already described in the Sanquhar and Leadhills districts ; for there we 

 find a sequence from the Arenig volcanic rocks and overlying Radiolarian 

 cherts and graptolitic mudstones with Glenkiln graptolites upwards to the 

 greywackes, shales and conglomerates of Llandeilo-Caradoc age. (Portandea 

 and Currarie north of Glen App.) 



North of the Stinchar valley, however, the sequence is wholly different. 

 One of the striking features of this northern area is the remarkable develop- 

 ment of Arenig volcanic rocks near Ballantrae, comprising diabase and diabase 

 porphyrite lavas, agglomerates and tuffs. The age of this volcanic series is 

 clearly defined by the occurrence in it of black shales and cherty mudstones 

 yielding graptolites of Lower or Middle Arenig age. North of the Stinchar 

 valley they are overlain by a great development of Radiolarian cherts 

 (Arenig and Lower Llandeilo) and other sediments. The lavas, tuffs and 

 overlying sediments are pierced by various intrusive igneous rocks, including 

 serpentine, gabbro, dolerite, diorite, and granite. Of special importance also 

 is the marked unconformability that separates the rocks just described from 

 the overlying sedimentary series of Girvan. For the massive conglomerates 

 (Kirkland and Benan) at the base of the latter series not only rest on the 

 eroded surfaces of the volcanic and plutonic rocks, but are mainly composed 

 of materials derived from the underlying platform. 



The general succession and subdivisions of the Girvan series, as worked 

 out by Professor Lap worth, are given in the following table, from which it 

 will be seen that there is a marked divergence in the lithological characters 

 of the strata from those in the Central Moffat region. In most of the sub- 

 divisions there is a great development of coarse sediments consisting of con- 

 glomerates, grits, greywackes, flagstones, mudstones, and shales, implying 

 deposition near land and in comparatively shallow water. As might be 

 expected from the foregoing conditions the Girvan series yields organic 

 remains in profusion, on certain horizons. Of special importance are the 

 bands of graptolite shales, yielding many of the zonal forms of the Moffat 

 region, which enabled Professor Lapworth to correlate the Silurian divisions 

 of the Girvan and Moffat areas. (See general Table, pp. 424, 425.) 



The list of fossils from the Girvan area, exclusive of the graptolites, has 

 been compiled from all obtainable sources, and is, in large measure, based on 

 the general list and on Mrs. Gray's list of fossils published in the Geological 

 Survey Memoir on " The Silurian Rocks of Scotland." Special thanks are 

 due to Mrs. Gray for supplying a note of recent additions to her list, which 

 now appear for the first time. Want of space has prevented the insertion of 

 more than one or two localities for each fossil. When two localities are 

 given, they represent the lowest and highest horizons for each form. 



In this list each division of the Silurian system is represented by a capital 

 letter, each sub-division by a figure. 



