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XXII.— The Geology of Ardrossan. By J. D. Falconer, M.A., D.Sc, F.G.S. 



(With Two Plates.) 



(MS. received January 26, 1907. Read February 18, 1907. Issued separately May 9, 1907.) 



CONTENTS. 



PAGE 



I. Introduction 601 



II. Stratigraphy 603 



III. Petrography 603 



(a) The Carboniferous Lavas and Tuffs . . 603 



PAGE 



(b) The Carboniferous Intrusive Rocks . 605 



1. The Castle Craigs Picrite 



2. The Baths Gate Dolerite 

 (c) The Tertiary Dykes . 



IV. Description of Plate II. . 



605 

 607 

 608 

 609 



I. Introduction. 



The town of Ardrossan is built largely upon the 15 -feet beach* which to the north 

 and south of the promontory is a well-marked terrace of sand and gravel, but on the 

 promontory itself is represented by a rocky shelf rarely exceeding 12 feet in height. 

 The promontory owes its origin to a great extent to the igneous rock of the Castlehill 

 and the Inches, which has offered more resistance to marine denudation than the 

 neighbouring sandstones. The railway to West Kilbride runs for some distance along 

 the 50-feet beach, while the 100-feet beach is represented by an irregular accumulation 

 of sand and gravel some distance inland. 



The geology is interesting both on account of the variety of sedimentary and 

 igneous rocks to be found within a small compass, and on account of the excellent 

 exposures of faults of many different types which may be studied on the shore at low 

 water. Of the latter, three are particularly interesting. The faults of late Palaeozoic 

 age which bound the Castlehill converge towards a centre of faulting in the vicinity 

 of Dykesmains. The more northerly brings down the lava of the Castlehill against 

 massive red sandstones whose stratigraphical position is about 400 feet above the 

 cornstones ; the more southerly truncates obliquely the margin of the Castlehill lava, 

 and turns both it and the lower limestones on end at the Inches. The great sinuous 

 N.W.-S.E. fault of the Inches, the Harbourback, and the Horse Island, probably of 

 Tertiary age, is more a great lateral wrench or shatter-belt than an ordinary fault. 

 There has been little vertical displacement, but, wherever exposed, it is characterised 

 by a line of breccia tion with small anastomosing dykes rising through the broken rock. 

 At the Inches it cuts the Castlehill fault and shifts the outcrop of the upturned lime- 

 stones 300 yards to the west. 



* " The Geology of North Arran," Mem. Geol. Sur., p. 140. 

 TRANS. ROY. SOC. EDIN., VOL. XLV. PART. III. (NO. 22). 85 



