W. M. Hutchings — On some Lake-District Rocks. 537 



more studied in detail they will be found to present more diversity 

 of type than is usually supposed. 



I will first note the occurrence of a variety of rock not previously 

 recorded in this district. It is a quartz-andesite, or dacite. It is 

 exposed on the ridge between Greenburn and Wytheburn, not very 

 far from Dunmail Raise, and near to a new wire-fence which bounds, 

 I believe, the property of the Manchester Waterworks. It is a 

 dark-coloured rock, on a newly-fractured surface of which are seen 

 numerous light spots of porphyritic felspars, and some of calcite, 

 with many grains of quartz, some of good size. It was the grains 

 of quartz which instantly attracted my attention to this rock when 

 a bit was chipped off it in passing the exposed crag. 



Under the microscope these grains of quartz are seen to be 

 corroded and eaten out into " bays and gulfs," and their nature as 

 true constituents of the rock, and not extraneous fragments, is 

 vouched for by their enclosures of ground-mass. None of them 

 show definite crystalline form. In addition to the larger porphyritic 

 grains, some very small ones are seen among the ground-mass, but 

 not abundantly. 



The many porphyritic felspars are fairly well preserved. They are 

 plagioclase, mostly well twinned, sometimes on both albite and 

 pericline systems. So far as may be inferred from the extiction- 

 angles of symmetrically-extinguishing twins, the chief felspar present 

 appears to be oligoclase. 



The porphyritic ferro-magnesian constituent appears to have been 

 mainly, if not wholly, biotite, which was abimdant. The outlines 

 and cleavages of the numerous large individuals are still quite 

 preserved, but great alteration has taken place into chlorite, epidote, 

 and indeterminable matter, probably partly carbonates and ferric 

 oxides. 



I have not met with any other andesite in the district in which 

 biotite can be recognized as present, or formerly present, nor am 

 I aware of any such rock having been recorded. My impression 

 is that biotite-andesite was very sparingly i-epresented, though of 

 course it is possible that the chlorite, so abundant in some cases, 

 may partly represent former biotite ; but in that case we should 

 expect to sometimes see recognizable pseudomorphs, as one so 

 frequently does in these rocks after augite. 



The ground-mass of this quartz-andesite consists of a network of 

 small felspar-laths, intermixed with, and at places more or less 

 obscured by, grains of chlorite, epidote, sphene, calcite, and other 

 matter, as is usual in all these rocks. In the greater portion of 

 it the felspars are very distinct and fresh, and allow of optical 

 measurements. They are largely untwinned, the rest being binaiy 

 twins. Extinctions are either quite parallel or at very low angles. 

 We may conclude that there is oligoclase, and most likely some 

 orthoclase present. So far as can now be made out, very little, 

 if any, glass was present in the original rock. There are many 

 good-sized crystals of apatite, and zircons are much more numerous 

 than in the usual local andesites. The sp. g. of the hand-specimen 



