32 GEOLOGY OF ARRAX. 



that is, composed of one kind of rock; a few only are com- 

 posed of parallel bands of different substances. By far the 

 greater number, in every part, consist of common trap that 

 is, some variety of greenstone or basalt. 



17. M. Necker, who visited Arran in 1839, carefully 

 measured the direction by the compass of a great number of 

 dikes seen along the eastern shore and in the interior, and 

 laying these directions down upon a map, he identified a 

 great many of them at remote points. Such continuity has 

 been often made out through distances greater than the whole 

 length of Arran ; but we know of few tracts pervaded by 

 dikes, where the same caution is required in drawing such 

 conclusions, on account of the great number of dikes, their 

 varying directions, and the undulations to which the same 

 dike is subject. The prevailing direction is towards the 

 north-west and north-east quarters, and nearly within the 

 limits between which the magnetic needle is known to vary. 

 Yet are there many which range without these limits, and 

 not a few have a direction nearly due east and west. Most 

 of the dikes are vertical; those which are inclined to the 

 horizon seldom pass an angle of inclination of 20. M. 

 Necker estimates the number of dikes between Loch Ranza 

 and King's-cross point at 200; the number to the south of 

 this,' on the east coast, at 144, making a total of 344. The 

 remaining portion of this estimate we give in his own 

 words : 



" Mais 1'evaluation precedente ne comprend que les dykes 

 de la surface d'une inoitiS environ de 1'ile; tout 1'int^rieur de 

 la partie meridionale n' y est pas compris, non plus que la 

 cote N.O., ni le groupe granitique de Ben Vearan entre cette 

 c&te et la rividre Irea; et quoiqu' il soit connu que 1'interieur 

 des terres renferme toujours moins de dykes que les cdtes, et 

 que la cdte N.O. est en g6n6ral tres d^pourvue de dykes, 

 quoiqu' enfin cette moitie de 1'Ile soit bien plus petite que 

 cette que j'ai parcourue; omettant ces circonstances je por- 

 terai pour elle un nombre gal a la premiere, soit 344, form- 



