226 MESSRS. B. N. PEACH AND W. GTJJSN ON [May IQOI, 



15. On a Eemaee:able Volcanic Vent of Tektiary Age in the 

 Island of Arran, enclosing Mesozoic Fossiliferous Rocks. — ■ 

 Part I: The Geological Structure, by Benjamin Neeve Peach, 

 Esq., P.R.S.,L. & E., F.G.S., and William Gunn, Esq., F.G.S. ; 

 Part II : Pal^ontological Notes, by Edwin Tcllet Newton, 

 Esq., E.R.S., E.G.S. (Read March 20th, 1901.) 



[Communicated by permission of the Director-General of 

 H.M. Geological Survey.] 



Part I. The Geological Structure. 



During the progress of the geological survey of the Island of 

 Arran Mr. Gunn mapped the network of igneous rocks that occurs 

 about halfway between Brodick Ba}'- on the east and Machrie Bay 

 on the west, and lying to the south of the String Road which crosses 

 the island from Brodick to Sbiskine and Blackwaterfoot (Sheet 21 of 

 the 1-inch Geological Survey Map). These rocks form the mass 

 of high ground culminating in Ard Bheinu (1676 feet), A'Chruach 

 (1679 feet), and Beinn Bhreac (1649 feet), whence the headwaters 

 of the Glen Cloy Burn and Benlister Burn drain to the east and the 

 Black Water and Machrie Water to the west. The area in which 

 these igneous rocks are exposed has obviously been a focus of great 

 volcanic activity. It may be compared with some of the larger and 

 more complex vents of Palaeozoic and Tertiary age in Scotland, but it 

 is exceptionally gigantic in its proportions. It is oval in shape, and 

 covers an area of about 7 to 8 square miles. Its greater axis, which 

 trends east- north-east and west-south-west, is 3g miles long, and its 

 shorter axis is about 3 miles in length. It is surrounded by various 

 formations, ranging in age from the Lower Old Red Sandstone to 

 the Trias. On the north side, conglomerates, sandstones, and mud- 

 stones of Lower Old Red Sandstone age, dipping steeply south- 

 eastward, strike obliquely against its northern margin. On the 

 east side, the Lower and Upper Old Red Sandstone, followed con- 

 formably by the various members of the Carboniferous system, 

 are truncated and altered by the igneous rocks. On the east side 

 also, these are followed by Triassic sandstones, resting unconforpaably 

 upon Carboniferous rocks, and, like them also, truncated and altered. 

 Along the southern margin, the intrusive rocks of the area abut 

 upon strata of the Lower Old Red Sandstone, which here dip south- 

 ward and eastward, and are profoundly modified in places near the 

 contact. On the west side, the Lower and Upper Old-Red-Sandstone 

 rocks and the Triassic sandstones and marls are truncated and 

 altered by the igneous rocks. The result of the detailed mapping 

 clearly shows that the igneous rocks, which are crowded together 

 within the area here described, must be later in date than the 

 present disposition of the surrounding sedimentary formations, and 



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