.382 



SIK A. GEIKIE ON THE TERTIARY 



[May 1896, 



Y. The Dykes. 



I have little to add to the full description already given by me 

 of the system of dykes which forms so important a feature in the 

 volcanic history of Tertiary time throughout the North-west of 

 Europe. It is difficult to establish any criterion of the relative 

 dates of protrusion of the dykes • but the important fact announced 

 by me so far back as 1857, that some are older and some later 

 than the great acid bosses of the Inner Hebrides, has been fully 

 confirmed by more recent research all over the region. So far as 

 may be inferred from the geology of the Red Hills of Skye and 

 their surroundings, the vast majority of the dykes belong to a time 

 anterior to the uprise of tbe bosses of granophyre. As an example 

 of the way in which these bosses truncate the dykes, I may cite 

 here a fresh illustration from the granophyre of Ben an Dubhaich, 

 near Torrin in Skye. The Cambrian limestones of that part of 

 Strath are traversed by numerous dykes which stop short at the edge 

 of the acid rock. As the actual lines of junction are not always 

 visible, it might be contended that the dykes are not necessarily 

 older than the granophyre, but may actually be younger, their 

 sudden termination at the edge of the acid boss being due to their 

 inability to traverse that rock. That this explanation is untenable 

 is readily proved by such sections as that given in fig. 26, where a 

 basic dyke 9 or 10 feet broad run- 

 ning through the Cambrian lime- 

 stone of Torrin is abruptly cut off 

 by the edge of the great grano- 

 phyre boss of Ben an Dubhaich. 

 Not only is the dyke sharply trun- 

 cated, but numerous pieces of it, 

 from 1 to more than 12 inches 

 in length, are enclosed in the 

 granophyre. 



Mr. Harker informs me that, 

 while carrying on the Geological 

 Survey of the district of Strath 

 (Skye), he has obtained data from 

 which it may be possible to deter- 

 mine certain broad distinctions 

 between dykes older and those 

 newer than the intrusion of the 

 granophyres. If these distinctions 

 are found to hold good, they may 



eventually be applicable to the elucidation of the relative ages of 

 dykes even at a distance from the granophyre, where nothing but 

 petrographical characters are available as a guide. 



Numerous basic dykes traverse the gabbros and granophyre of 

 St. Kilda. Those in the former group of rocks are more abundant 

 than those in the latter — a circumstance which is exactly paralleled 

 among the basic and acid bosses of Skye. It is not improbable 



Fig. 26. — Ground-plan of basic 

 dyke (b) in Cambrian lime- 

 stone (a) truncated by grano- 

 phyre (c) which encloses large 

 blocks of the dyke. Torrin, Skye. 



