82 



composed of one kind of rock ; while a few, of which the most re- 

 markable are those of Tormore, are composed of parallel bands of 

 different substances. By far the greater number, in every part, con- 

 sist of common trap, that is, some variety of greenstone or basalt. 



57. 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 varying directions of the dikes, 

 and the undulations to which the same dike is subject. The pre- 

 vailing 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 num- 

 ber of dikes between Loch llanza 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 moitie environ de 1'ile; tout 1'interieur de la partie 

 meridionale n' y est pas compris, non plus que la c6te N. 0., ni le 

 groupe granitique de Ben Vearan entre cette c6te et la riviere Irsa ; 

 et quoiqu' il soit connu que 1'interieur des terres renferme toujours 

 moins de dykes que les cotes, et que la c6te N. 0. est en general 

 tres depourvue de dykes, quoiqu' enfin cette moitie de 1'ile soit bien 

 plus petite que cette que j'ai parcourue ; omettant ces circonstances 

 je porterai pour elle un nombre egal a la premiere soit 344, form 

 ant un total de 688, ou, en nombre rond, de 700 dykes de trap dans 

 la totalite de 1'ile de Arran. Doublant meme encore ce nombre si 

 Ton vouloit, pour y comprendre tous les dykes caches par les bruy- 

 eres vastes et etendues dans 1'interieur, par les greves de sable sur les 

 rivages, ou places dans des recoins inaccessibles des montagnes, on 

 n'arriveroit pas encore au nombre de 1,500 ; et pourtant en parlant 

 de telle ou telle cote, de telle localite d'Arran, il est souvent echappe 

 a ceux des geologues qui ont decrit Arran, a moi-meme peut-etre 

 tout le premier, de dire qu' on y voyoit des innombrables dykes de 

 trap. Or, je crois avoir maintenant montre" que, loin de ne pouvoir 



