OLDER PERIDOTITES OF SCOTLAND. 399 



identical with those arrived at in this paper concerning the rocks in 

 question. 



An equally striking example of a rock of the same class occurs in 

 Forfarshire, near the town of Kirriemuir. This rock' was described 

 in the year 1825 by Sir Charles Lyell, after studying it with the 

 assistance of Dr. Buckland, as a mass of serpentine forming a dyke 

 which intersects the Old E-ed Sandstones and contemporaneous 

 volcanic rocks of the district *. In 1875 I had the advantage of 

 studying this mass of serpentine under the guidance of Sir Charles 

 Lyell. The dyke of serpentine can be traced running for a length 

 of at least 14 miles, in an E.N.E. and W.S.W. direction, near the 

 southern foot of the Grampians, and parallel to that range ; it is 

 well exhibited in several deep cavities, cut by streams descending 

 from the mountains, especially those of the Carity, the Prosen, and 

 the South Esk. The dyke varies in width at different points from 

 100 to 300 yards ; it encloses " horses " or masses of the rocks 

 traversed by it, and is itself intersected by other intrusive rocks. 

 It produces marked alteration on the rocks which it traverses f. 



At its sides the rock is a mass of serpentine, traversed by nume- 

 rous veins of chrysotile and exhibiting no evidence of the minerals 

 of which the rock was originally formed. But towards the centre, 

 crystals of " Schiller spar " make their appearance, and the serpen- 

 tine gradually passes into a hard crystalline mass which Lyell com- 

 pared with the rock of the Cuchullin Hills, and called hypersthene- 

 rock. 



Studied by the aid of the microscope, this central and least 

 weathered part of the dyke is seen to be made up of serpentine, 

 clearly pseudomorphous after ohvine, and containing large crystals of 

 a ferriferous enstatite in a more or less advanced stage of alteration 

 into bastite and serpentine. In some portions of the central mass 

 of the dyke the ferriferous enstatite prevails almost to the exclusion 

 of the olivine, and we have a rock strikingly resembling the bronzite- 

 rock of the Kupferberg, near Bayreuth, and of St. Stephan in Upper 

 Styria (see PI. XIII. fig. 7). This is a type of rock which has 

 not, I believe, been hitherto recognized in the British Islands. As 

 we trace the rock outwards from the central mass, the alteration 

 becomes greater, till at last all traces of the individual enstatite- 

 crystals disappear, and we have a serpentine in which no vestige of 

 the original mineral constituents of the rock can be distinguished. 

 Among the dykes which intersect the great serpentine-mass, I found 

 one to consist of a coarse dolerite or augite-gabbro, while another 

 is a very beautiful example of a hypersthene- (ferriferous enstatite) 

 dolerite. 



The serpentines with altered bronzite, from the neighbourhood of 

 Aberdeen J, are also altered olivine-enstatite rocks. 



When the older peridotites contain a considerable proportion of 



* Edinb. Journ. of Sci. vol. iii. (1825) p. 112. 



t I am much indebted to Leonard Lyell, Esq., F.G.S., of Kinnordy, for 

 several interesting series of specimens which he has sent me from this district. 

 I See Heddle, Mineralog. Mag. vol. v. pp. 4-6. 



