64 GEOLOGICAL FEATURES OF BUTESHIRE. 



of Muchrie "Wa,ter. Between this point and Dougrie, near Iorsa 

 water foot, it attains its greatest breadth. A line from the north side 

 of Brodick Bay to Dougrie very nearly marks out its line of junction 

 with the slate. 



The southern half of the island, southward of the Old Bed Sand- 

 stone boundary above defined, that is, from the String Boad and 

 " of Muchrie "Water to the south end, is composed of several 

 members of the Carboniferous series, broken through and overlaid 

 by various igneous rocks, chiefly those of the greenstone type. The 

 prevailing rock and substratum of the whole southern plateau is red 

 sandstone, varying from a fine compact structure to that of a coarse 

 conglomerate. A band of this sandstone extends also along the eastern 

 shore northwards to Corrie, and again from the Fallen Bock? north- 

 3a :den. at the northern extremity near the entrance of 

 Loch Banza. Subordinate to this sandstone are beds of limestone 

 abounding in fossils of true carboniferous types, with beds of shale 

 and coal, in which are found fossil plants and shells such as charac- 

 terise the.^e strata in the basin of the Clyde. 



Arian is extremely rich in rocks of the trapean order; most of 

 the known species occur, and also those numerous varieties by which 

 they graduate into one another. They form great overlying masses, 

 capping the sandstone of the southern plateau, and rising into the 

 highest hills of this division of the island. They are interposed 

 amid the sedimentary deposits, in huge sheets or beds conformable 

 to the stratification, and cut through all the rocks alike, from the 

 1 jwe.rt to the highest in vertical or slightly inclined dikes, which 

 range continuously across great horizontal distances. 



The trap rocks of Arran may be arranged in three classes, accord- 

 • their composition : — The Felspathic, comprising porphyry, 

 clay-tone, compact felspar, and pitchstone; the Hornblendic, as 

 diorite and amygdaloid ; the Augitic, as basalt and dolerite. 



mbrae. — This island is three miles long, and one and a- 

 half broad. The surface rises gently towards the middle, and the 

 height nowhere exceeds 500 feet; the southern shore presents some 

 low rocky cliffs ; on the other sides the ground descends by grassy 

 slopes and sandstone ledges. Great Cumbrae corresponds in 

 geological structure with the middle region of Bute, and consists of 

 red sandstone having a slight northerly dip, traversed by numerous 

 trap dikes, invaded by sheets of the same rock, but not overlaid by 

 trap except in one spot on the west coast, where a small patch occurs. 

 The sandstone is generally of a deep red colour, almost the only ex- 



