Me. J. Napier on Trap Dykes, Island of Arran. 323 



part of the bed of trap, and the portions of sandstone still left with 

 the cracks give to the radiating dykes the appearance of coming from 

 this bed. Whether this may be the true explanation of these 

 phenomena remains for further inquiry; but one thing is certain, 

 that many of these apparent dykes are separated from each other by 

 mere pieces of stone lying upon and imbedded in the underlying trap 

 while in fusion. 



The whole results of the continued observations on these dykes do 

 not lessen my belief in the opinion stated in the former paper, as to 

 the dykes being cracks from a central upheaval of large masses — not 

 that all dykes are thus formed, for a dyke may be produced in a I'ock 

 without any central upheaval ; but that all sudden upheavals of trap 

 will be foimd surrounded by a number of dykes radiating less or more 

 in a straight line from the centre. 



Owing to many of the dykes branching off into several smaller dykes, 

 the number seen upon the water line will vary according to the different 

 states of the tide, and the line taken by the observer, which will account 

 for the difference mentioned in Dr. Bryce's work on Arran, of the dykes 

 enumerated between Brodick and Lamlash by Philips, Necker, and my- 

 self. If we take the gross number visible, with all their branches, between 

 low water mark and the soil, there will be found more dykes from Glen 

 Sannox to Whiting Bay than M. Necker estimates for the whole of 

 the Island. The fact that in a stripe of a few miles and a few 

 yards broad, there will be seen not less than 300 dykes, varying in size 

 from one foot to 100 feet, gives the whole subject an attractive interest ; 

 and when we find not less than 95 per cent, of the whole running towards 

 the hills, or gi'eat upheavals, we are warranted to conclude that there 

 must be an intimate connection between them.* 



Many of the large boulders at Kingscross Point are penetrated 

 by Uttle dykes of a fine-grained trap. This may have been caused 

 by sudden cooling over the surface, producing small cracks, which 

 have been filled up even during the same upheaval. The internal 

 structure of trap rocks is greatly influenced by the rate of cooUng 

 to which they have been subjected while under the pressure of the 

 solid crust, before being upheaved, allowing the formation, in the 

 fluid mass, of different compounds, forming into distinct crystals ; and 

 the rock, when solidified, would thus acquire an amygdaloid or por- 

 phyritic character, which may be defined by reference to some 

 analogous phenomena. 



* To embrace the idea that the direction of these dykes has a relation to the magnetic 

 line, as has been stated by Necker, would necessitate the belief that this line embraces 

 the whole round of the compass. 



