Vol. 60.] ROCKS OF THE BORROWDALE VOLCANIC SERIES. 101 



in the ' streaky ' rocks seems to be an original feature. A study, 

 both microscopical and chemical, of these lavas is necessary, before 

 it can be asserted that the Eycott Group is associated with the 

 'streaky' rocks in one area and that it is older than them in another. 



IX. The Characters of the Garnets. 



These are of no very great interest from the crystallographic point 

 of view. In the intrusive rocks and the majority of the ' streaky ' 

 rocks they do not usually show good faces. The well-developed 

 garnets are found in abundance in the ashes of Illgill Head, High- 

 Goat Gill (Haweswater), and Sty-Head Tarn. 



The only two forms met with are the icositetrahedron (211) 

 and the dodecahedron (110). The icositetrahedron is occasion- 

 ally found singly, but I have never met with the dodecahedron 

 alone. The majority of the garnets show a combination of the two 

 forms, the relative size of the respective faces varying considerably. 

 Garnets a quarter of an inch in diameter are quite common, but 

 the extreme limit is half an inch. 



The occurrence of a ring of felspar round the garnet, with the 

 production of ferromagnesian mineral and separation of iron-ore, 

 has already been considered (pp. 73, 91). The conversion of the 

 garnet into a sericitic substance by dynamic metamorphism has also 

 been noticed (p. 88). 



The garnets found in the greenish rock of High-Goat Gill, a 

 tributary of Xaddle Beck (Haweswater), show somewhat peculiar 

 features. This rock is an andesitic ash, containing many felspar- 

 crystals in a greenish matrix charged with chlorite. The silica- 

 percentage is 56*45. The garnets are extremely abundant and 

 almost perfect in outline, but usually show a small indentation 

 occupied by the greenish matrix. This cavity may occur anywhere 

 on the crystal, and has no relation to the crystallographic axes. 

 It may be either five- or six-sided, according to the number of 

 faces which it cuts, but it is often roughly circular in outline. 

 A series of step-like projections running round the cavity occur all 

 the way down to the bottom, and seem to represent the layers of 

 garnet-growth. This phenomenon is singularly suggestive of a 

 metamorphic origin for the aarnets. The rock is an undoubted 

 ash, therefore the cavities could not be formed by corrosion. There 

 may be two such cavities in one garnet, and nearly all the garnets 

 show the same phenomenon. It is extremely improbable that such 

 hollows would be found in fragmental garnets. If the garnets are 

 metamorphic, it is difficult to understand how such metamorphism 

 has been produced, for the rock is far removed from an intrusion 

 of any great magnitude. 



Chemically, the garnets are ii'on-alumina garnets, with small 

 quantities of calcium, magnesium, titanium, and manganese-oxides, 

 and therefore belong to the alman dine- type. This was proved 

 by qualitative analysis, and by quantitative estimation made on 

 those garnets which could be procured in some quantity with 

 little trouble : these are to be found only in the ashes at the 

 upper and lower limits of the ' streaky ' rocks. To obtain 



