225 



or less changed to leucoxene along narrow parallel bands, and a little 

 quartz and chlorite. In this groundmass lie aggregates of irregular 

 augite-individuals, often frequently twinned, and serpentinous pseudo- 

 morphs after olivine or enstatite, probably the latter. 



The rock from Swirral Edge is mentioned by Professor ROSENBUSCH 

 as a typical leucophyre. (1) GUMBEL'S definition of this term has been 

 already quoted (p. 135). ROSENBUSCH considers that the distinguishing 

 characteristics of the group must be looked for in the large excess of 

 plagioclase over augite and the entire absence of magnesia-mica and 

 hornblende. Quartz is generally present and is in part, if not entirely, an 

 alteration product. Calcite and chlorite are very common. A specimen 

 from Swirral Edge collected by the author shows under the microscope 

 a large amount of turbid felspar, indistinct pseudomorphs of calcite dust 

 and chlorite after augite, probably ophitic, and brownish leucoxene plates 

 after ilmenite. In the hand specimen the rock is pale in colour for a 

 diabase and therefore in this respect answers to GUMBEL'S definition 

 of leucophyre. 



The typical ophitic diabases, so common in North Wales, have not 

 as yet been recognized as occurring in the Lake District in any quantity. 

 A rock of this character is found, however, at Walla Crag, Haweswater. 

 Under the microscope the most conspicuous mineral is a nearly colourless 

 pyroxene which occurs in large ophitic plates. Next in importance are 

 greenish pseudomorphs of fibrous structure. These occur independently 

 in the rock and also as inclusions in the augite. Under crossed nicols 

 they split up into isotropic or nearly isotropic and vividly polarizing 

 portions. The latter form a very irregular network, and the isotropic 

 portions form the centres of the meshes. The vividly polarizing portions 

 extinguish simultaneously. These are in all probability pseudomorphs 

 after a mineral belonging to the enstatite-group. The lath-shaped felspars 

 are highly altered and give now only aggregate polarization. Titaniferous 

 iron ores and apatite are present. The rock appears, therefore, to be an 

 ophitic enstatite diabase. 



The lavas of the Lake District appear to be as a rule of intermediate 

 composition (altered andesites). Mr. WARD has, however, called 

 attention to the existence at Eycott Hill of a fine series of lava flows 

 and ash-beds of a somewhat basic character (51 to 53 per cent, of silica). 

 Professor BONNET < 3 > has recognized the existence of a rhombic pyroxene 

 in these rocks. Mr. WARD indicates fourteen distinct flows in his section. 

 The second of these, reckoning from the base of the section, is the most 

 remarkable. It is vesicular in its upper portion and the vesicles ' are 

 drawn out along the line of flow. They are now filled with chlorite 

 chalcedony and quartz. The main mass of this rock is rendered 



(1) Massige Gesteine. Second Edition. 1886. p. 200 



(2) Q,. J. G. S., Vol. XXXI., p. 406 ; Microscopical Journal, 1877, p. 239. See also Survey 

 Memoir on Sheet 101 S. E. 



(3) G. M., 1885, p. 76, 



