provided we recognize their true characters and relations. They con- 

 sist essentially of plagioclase, augite (mostly in grains) and magnetite, 

 with a variable and sometimes a not inconsiderable amount of brown 

 crypto-crystalline interstitial matter. Their silica percentage is higher 

 (sometimes 59 per cent.), and their specific gravity lower (about 2'7 or 

 2*8) than the corresponding features in normal dolerites and 

 basalts. In colour and general appearance they resemble the dolerites 

 and basalts, but in composition they are somewhat allied to the 

 andesites. Professor ZIRKELW called special attention to the existence 

 of quartz in some of these rocks. It is difficult to be certain as to the 

 nature of this quartz. In some cases, as for example, in a dyke occurring 

 to the North of Drumadoon Point on the West Coast, it appears to be 

 a foreign ingredient. It occurs in rounded grains and each grain is sur- 

 rounded by a zone which suggests that the magma has exerted a corrosive 

 action upon it. The Tertiary dykes of the South of Scotland and North of 

 England are for the most part andesitic dolerites. M. BRE'ON has described 

 somewhat similar rocks from Iceland as augite- andesites, and proved that 

 they are later in date than the more widely distributed olivine-dolerites. 

 The Heckla lava of 1845 is a rock of this type. Dr. GEIKIE has shown 

 that the dykes of the West of Scotland frequently intersect the older 

 sheets. Taking all these facts into consideration, it appears that the 

 andesitic dolerites are later in date than the olivine-dolerites in the Brito- 

 Icelandic province and that many of the great dykes belong to this 

 later period. 



The olivine-dolerites of Arran, so far as they have been examined, 

 possess the ophitic structure in the greatest perfection. They occur in 

 the sheet capping the Clauchland Hills and Dun Fion and in the south- 

 ern portion of the Island between Dippin Point and Benan Head. Mr, 

 ALLPORT describes the occurrence of three distinct sheets of dark trap 

 near Kildonan Castle. The middle one is an ophitic dolerite of the 

 Dun Fion type. The lowest and uppermost sheets are of andesitic dolerite. 

 In the Allport collection in the British Museum there are sections of 

 four dykes ; one from the neighbourhood of Corrie, another from Glen 

 Shirrag, a third from Torlin Water, and a fourth from the neighbourhood 

 of Drumadoon Point. These are all sections of andesitic dolerites and bear 

 the closest resemblance to sections of the Eskdale, Acklington, and Cleve- 

 land dykes. M. NECKER < 8 > estimated the number of dykes between Loch 

 Ranza and King's Cross Point at 200, and the total number on the Island 

 at over 1,000. 



Of the numerous N.W. and S.E. dykes in the southern portion of 

 Scotland only one, that of Eskdale, has been described in detail.< 4) It is 

 an andesitic dolerite (augite-andesite), composed of lath-shaped plagio- 



(1) Geologische Skizzen von der Westkuste Schottlands, Z.D.G.G., Vol. XXIII. p. 28. 



(2) Geologic de 1'Islande, Paris, 1884. 



(3) Documents sur les Dykes de Trap d'une Partie de 1'ile d' Arran. Trans. Roy. Soc. Edin., 

 Vol. XIV., Part II., 1840, p. 684. 



(4) Dr. GEIKIE. The " Pitchstone " of Eskdale. Roy.Phy. Soc. Edin., Vol. V., 1880, p. 219. 



