352 G. W. Tyrrell— Tertiary Dykes 



The Ayrshire examples of the Brunton type of tholeiite may be 

 represented by a N.N. W". dyke crossing the River Ayr near Stairaird, 

 Mauchline. This rock (Fig. 2a) shows clusters of almost colourless 

 granular augite, intermixed with diversely arranged, short felspar 

 laths (Abi AUi), all held in a black glass. The phenocrysts consist 

 of bytownite-anorthite with a narrow rim of labradorite, exactly 

 similar to those of cumbraite. A rough estimate of relative 

 proportions is about 35 per cent each of the groundmass felspars and 

 augite, 20 per cent glass, and 10 per cent of anorthite phenocrysts. 

 A very small amount of olivine, fresh in this rock but serpentinized 

 in the others, may be detected. On examination by a high-power 

 objective the " black " glass turns out to be colourless, but charged 

 with swarms of opaque black microlites. N.N.W. dykes conforming 

 to this type, and exhibiting only minor variations in the quantitative 

 relations of the minerals, occur in the River Ayr at Failford, near 

 Tarbolton ; in the Broadlie House Burn, near Dairy; and in the 

 Coachford Burn, Cooperhill, Ochiltree. 



Tholeiites of the Brunton type do not appear to be common in 

 Arran, only three examples being known in a large suite of the 

 Tertiarydykes. These dykes occur in the South Locherim Burn, Come; 

 at 1,538 feet on the Saddle, between Glen Rosa and Glen Sannox; 

 and on the shore near Largiemeanoch, Whiting Hay. Likewise 

 Dr. W. R. Smellie's slides of the Tertiary dykes of Bute show only 

 three examples belonging to the Brunton type, a dyke at the south 

 end of Loch Fad; in the quarry behind the Power Station, Ardbeg, 

 Rothesay ; and on the shore south side of Stravannan Bay. Only one 

 dyke of the Brunton type occurs in the Great Cumbrae, a north-west 

 dyke, 1 1 feet thick, at Portachur Point. 



Olivine Tholeiite. — These dykes appear to represent a further 

 and more basic stage than those of the Brunton type, although 

 preserving a sufficient community of character with the foregoing to 

 warrant the view that they are a continuation of the same series. 

 They are characterized by the presence, often an abundance, of fresh 

 olivine, and by the presence of a glassy base in minimum quantity. 

 No chemical analysis of this group has yet been made. Two types 

 are recognized according to the relative abundance of olivine. The 

 first type, characterized by a small quantity of olivine and by its 

 restriction to the groundmass, is represented by a large group of 

 dykes near Corrie, Arran, and may accordingly be known as olivine 

 tholeiite of the Corrie type. A typical example is provided by 

 a dyke at Birchpoint, on the shore south of Corrie. This rock 

 (Fig. 2b) carries numerous small phenocrysts of bytownite-anorthite 

 with narrow labradorite margins. These crystals are often worn 

 and corroded into curiously irregular shapes, and have glassy 

 inclusions arranged in a thick marginal zone which faithfully follows 

 the fantastic outlines of the crystals. The groundmass consists of 

 a diverse mesh of short laths of labradorite (AbiAm), intermixed 

 with granular or short prismatic crystals of a pale, violet-brown 

 augite which is the most abundant constituent of the groundmass. 

 Minute grains of olivine, partially or wholly serpentinized, and of 

 euhedral magnetite, are uniformly scattered over the field, but form 



