145 
THE ANCIENT 
LAVAS OF THE ENGLISH LAKE DISTRICT. 
ALFRED HARKER, M.A., F.G.S. 
THE existence of a great series of volcanic rocks in the English 
Lake District has long been recognised, although very little precise 
information about them has been forthcoming. The rocks were 
first described by Sedgwick, who, in the terminology of his day, 
named them the ‘Green-Slates and Porphyries.’ Messrs. Harkness 
and Nicholson have termed them the ‘ Borrowdale Series,’ from the 
fine development of a portion of the succession in that valley. The 
series forms part of the Ordovician system (Lower Silurian of the 
Geological Survey), and must be of the same general age as 
the volcanic rocks of Caernarvonshire. The volcanic accumulations 
Occupy all the central part of the Lake District, being bounded 
northward by the Skiddaw Slates, while to the south they terminate 
with the Coniston Limestone, and are succeeded by the Silurian 
Strata (Upper Silurian of the Survey). Further, a belt of volcanic 
rocks belonging to the same series runs along the northern edge of 
the Skiddaw Slate area. 
Fragmental rocks (breccias, coarse and fine ashes, etc.) form a 
large part of the volcanic series, often in beds alternating with the lavas, 
but it is to the latter alone that we shall refer here. With regard to 
chemical composition, the lavas divide into three distinct sets—basic, 
intermediate, and acid; and these will be briefly noticed in order. 
The éasic davas, wens certain associated ashes, etc., occupy the 
northerly belt already mentioned, and are especially well exhibited 
at the eastern end of their course, in Eycott Hill, near Troutbeck 
Station. They contain 51 to 53 per cent. of silica, and have 
@ specific gravity of about 2°75. Unlike most lavas of similar 
chemical composition, they contain no olivine, but instead we find 
other basic minerals abundant, and particularly hypersthene; so 
that the rocks may be termed hypersthene-basalts. The most 
striking type is seen in a thick flow rather low down in the Eycott 
section, where it stands out conspicuously, forming a row of small 
prominences. It is distinguished by its enclosing porphyritic crystals 
of triclinic felspar up to an inch or even two inches in length in 
a compact ground. The crystals often have a rather tabular shape, 
with rounded outline. | When fresh, they are quite glassy in aspect, 
and show distinctly the Carlsbad twinning, as well as the usual 
twin-striation. They belong to the variety bytownite. The other 
flows seen in Eycott Hill are compact throughout, or enclose only 
May 289r, K 
