THE ROCKS OF THE CLYDE DRAINAGE AREA. 

 BY JOSEPH SOMMERVILLE AND G. R. THOMPSON. 



THE Rocks of the district are given in a tabulated form, and are chiefly 

 compiled from Sir Archd.Geikie's "Ancient Volcanoes of Britain," the Memoirs 

 of the Geological Survey of Scotland, and the Transactions of the Geological 

 Society of Glasgow. 



IGNEOUS ROCKS. 



The Igneous Rocks have been classified from their chemical and mineralo- 

 gical composition. For their positions in geological chronology, see "The 

 Ancient Volcanoes of Britain." They are divided into four groups as 

 follows : 



Acid group. Those containing over 65 per cent, of silica. 

 Granite Arran : Loch Doon : Spango. 

 Micro-Granite, Eurite or Granulite Ailsa Craig. 

 Graphic Granite or Pegmatite Cowal District. 

 Felsite Quartz-porphyry and Feldspar-porphyry or Porphyrite Tinto : 



Lochfyne : Ballantrae. 

 Pitchstone Arran. 

 Intermediate group. Those containing 55 to 65 per cent, of silica. 



Lamprophyre, including Kersantite, Camptonite, Vogesite, etc. Cowal 



District. 



Diorite Girvan and Cowal districts. 

 Trachyte in dykes in the Cowal district. 

 Andesite largely represented in the district and often a.mygdaloidal. 



Dumbarton, Ayr, Renfrew, and Lanark shires. 

 Basic group. Those containing 45 to 55 per cent, of silica. 

 Gabbro : Hornblend gabbro and Hyperite Girvan : Cowal. 

 Basalt common in the district in intrusive sheets and dykes, often 



columnar. 



Tachylyte a basalt glass forming selvages of basalt dykes Whiteinch : 

 Lendalfoot. Mr. Clough has found it in the centre as well as on the 

 selvages of dykes ; see Survey Memoir: "The Geology of Cowal." 

 Dolerite commou all over the district in dykes and intrusive sheets. 

 Where it has been erupted or intruded amongst carbonaceous strata 

 its usual dark colour has been changed into grey and white. 

 Diabase and Diabase porphyrite, showing pillow structure Girvan dis- 

 trict : Dumgoyn. 



Ultra-Basic group. Those containing generally less than 45 per cent, of silica. 

 Serpentine Lendalfoot : Innellan. 



In this group may also be noted Olivine-enstatite rock and Horn- 

 blend-picrite Girvan district. 



VOLCANIC FRAGMENTAL ROCKS. 



Volcanic agglomerate Dunibuck : Campsie Hills : Renfrewshire and Ayr- 

 shire. 



