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volcanic origin; but there can be not a shadow of a doubt, judging 

 from its mode of occurrence, that the existence of the mineral in 

 that instance is due simply to the action of metamorphism. The 

 rock where the Idocrase occurs belongs to the upper or newer 

 group of volcanic rocks of the Lake District, associated with the 

 Coniston Limestone as contemporaneous members of the series. 

 These, like the much-more-fuliy-developed series exposed on the 

 same geological horizon on the south west flanks of Roman Fell,* 

 which I have proved to be of Bala age, are altogether distinct 

 from the Borradale Rocks proper. The occurrence of the mineral 

 in Westmorland was first made known by Professor Harkness, 

 whose large series of specimens illustrating its mode of occurrence, 

 now, together with most of his minerals, forms part of the 

 increasingly-valuable collection in the Carlisle Museum. Idocrase 

 occurs elsewhere in the British Isles as a product ef metamorphism; 

 and it is interesting to note that at least in same of the cases, the 

 rock metamorphosed has been, as at Wastdale Head, more or less 

 calcareous. 



The variety of Andalusite known as Chiastolite is a rock- 

 constituent of considerable importance in Cumberland. It occurs 

 exclusively as a product of metamorphism, resulting from the 

 alteration of the sedimentary beds of the Skidda Slate around 

 the granitic intrusions of the Cawda area. Mr. Ward has given 

 us detailed descriptions of the changes of lithological character 

 observable as the unaltered Skidda Slates are traced in the direction 

 of the granite. One of the manifestations of the regional meta- 

 morphism affecting that area is the sealing-up of the cleavage that 

 elsewhere affects these rocks.t Just outside the zone where this 

 phenomenon is observable, the crystals of Chiastolite attain their 

 fullest development. Within this zone, (in other words, nearer to 

 the granite,) the crystals of Chiastolite by degrees give place to 

 Mica, and the rock then passes from Chiastolite Slate into a kind 

 of Mica schist. It is noteworthy in this case that, in the stage of 



* See the collection of specimens in the Rock Collection of the Museum of 

 Practical Geology, where these rocks have been publicly exhibited and labelled 

 " Bala Volcanic Series" since the early part of 1881. 



t This fact does not appear to have been previously noticed. 



