76 Prof. T. G. Bonney — TJte Ensiatitic Lavas of Eijcott Hill. 



attention to the subject, and cannot find any limits (taking all Europe 

 into consideration) to its range within the Lower Carboniferous rocks. 

 In England, however, it has never yet been recorded from beds of 

 Tuedian age, nor, taking into account the nature of the Tuediaa 

 deposits, is it very likely that it may ever be found in them. 



V. — On the Occurrence of a Mineral Allied to Enstatite 

 IN THE Ancient Lavas op Eycott Hill, Cubiberland. 



By Professor T. G. Bonney, D.Sc. LL.D., F.R.S., Pres. G.S. 



THIS interesting series of ancient volcanic rocks is described by 

 the late Mr. Clifton Ward in a jiaper on the Microscopic Struc- 

 ture of Ancient and Modern Volcanic Rocks, read before the 

 Geological Society,^ in a Memoir of the Geological Survey on the 

 Lake District, and, in greater detail, in a communication to the Eoyal 

 Microscopical Society.^ All these are illusti-ated by figures (in no 

 case very good) and some chemical analyses are given in the last- 

 named paper. I went to the hill in the autumn of 1874 and 

 collected a few specimens, but my visit was cut short by heavy rain. 

 A few weeks since Mr. J. Postlethwaite of Keswick, to whose kind- 

 ness I have been more than once indebted for additions to my 

 collection, forwarded to me three specimens from Ej'cott Hill, 

 thinking that I might not have any rocks therefrom, and called my 

 attention to the peculiar reddish tint of the felspar in one of them, 

 which, as he remarked, " resembled the colour of a garnet." These 

 were varieties of the well-known porphyritic lava of Eycott Hill, 

 which is described by Mr. Ward as the second lava bed in ascending 

 order, and as being above 100 feet thick. This specimen was 

 obtained from a boulder on the hill. On examining it and com- 

 paring it with my own (at which I had not looked for some years), 

 I was struck with the appearance of the ground-mass, which seemed 

 to me unusually compact and more like that of an augite-andesite, 

 than of a doled te or diabase, as the rock is named by Mr. Ward. I 

 had a slide prepared by Mr. Cuttell from Mr. Postlethwaite's 

 specimen, in which I first discovered the mineral which is described 

 in this note, and have since had some more cut from my own speci- 

 mens from Eycott and others subsequently sent by him, and (for 

 comparison) from the lavas of Falcon Crag near Derwentwater. 



The Eycott rock is described by Mr. Ward as having " a compact 

 greenish-blue base, containing dark green spots of a soft mineral and 

 large porphyritically imbedded felspar crystals, many of them an 



inch long The top of this lava is beautifully vesicular in 



parts, the vesicles being drawn out along the line of flow and filled 

 with chlorite, chalcedony, and calcite." The " dark green spots " I 

 may add generally do not exceed -1 inch in diameter, and 1 should 

 be disposed to call the base a dark "slate " colour, commonly with a 

 greenish tinge (slightly purplish in Mr. Postlethwaite's specimen), 

 rather than a " greenish-blue " ; the felspar crystals are in the 

 normal specimens a greyish olive-green. 



^ Q,J.G.S. vol. xxxi. p. 406. ^ Moatlily Microsc. Journal, 1877, p. 239. 



