177 



land and Westmorland. So far as our present knowledge goes, 

 Augite does not occur as a product of metamorphism here, though 

 it is known to do so in other parts. Augite occurs in scattered 

 crystals, usually of small size, in the lavas of the Borradale volcanic 

 series — a fact of considerable interest and importance, if only as 

 proving that volcanic action at one of the remotest-known periods 

 of the earth's history was essentially the same in character as can 

 be actually seen in active volcanoes at the present day. Augite 

 occurs in the two counties also as a rock-constituent of some of 

 the plutonic rocks of later date than the Silurian series. It is 

 found again in rocks known to be contemporaneous with the older 

 part of the Carboniferous rocks ; as along the Border near Bew 

 castle. In the well-known Whin Sill again, which is a post- 

 carboniferous rock, it occurs as a representative of a period later 

 still ; while in the Armathwaite Dyke, the Augite constituent 

 occurs in one of the latest manifestations of plutonic action within 

 our district. This dyke is probably of Miocene age, as Dr. Geikie 

 has shewn is the case with some Scottish dykes occurring in the 

 same manner. 



DiALLAGE is another mineral closely allied in chemical composi- 

 te the three minerals just enumerated. It crystallizes in nearly 

 the same form as Augite ; but differs from that mineral in displaying 

 very perfect cleavage along planes parallel to one of the crystalline 

 faces. Some of this mineral shews cleavage faces with a sub- 

 metallic lustre, something like Hypersthene. It is, like Augite, 

 exclusively confined to eruptive rocks, at least in Cumberland and 

 Westmorland ; but it is of much less common occurrence than 

 Augite ; and, so far as is at present known, it does not occur here 

 in any rocks of later date than those of the Older Palaeozoic series. 

 Well-developed crystals occur in the beautiful rock known as 

 Gabbro, that occurs as an intrusive sheet in the metamorphosed 

 Skidda Slates at Cuns* Fell, near Ousby, as well as in the similar 

 rock forming Wallat Crag, Hawes Water. In both of these the 



* Welsh, Cwn — a hill. 



t There is a Swedish word Walla, meaning "grassy"; hence the spelling 

 "Wallow," which is sometimes used, is probably as little warranted by the 

 meaning of the word as it is by its local pronunciation. 



]3 



