88 MR. E. E. WALKER ON" THE GARNET-BEARING [Feb. I904, 



Numerous small garnets may be observed in the ash, but unless 

 specially looked for they would escape notice. 



The Frith -Wood breccia is intensely cleaved, and the garnets 

 show an interesting transformation. They are seen to consist of 

 a collection of irregular fragments, surrounded by a colourless 

 substance of indefinite outline. Between the fragments flakes of 

 what appears to be a colourless mica are developed ; chlorite-flakes 

 are also present, and possibly quartz is set free along with rods 

 and irregular masses of ilmenite or other iron-ore. 



This peculiar change takes place in all cleaved garnet-bearing 

 rocks : the garnet-fragments become smaller and finally disappear, 

 leaving only a few grains of iron-ore and an indefinite sericite-like 

 growth. Examples of this change are afforded by the cleaved 

 intrusive rock of Dock Tarn previously described (p. 85), by a 

 cleaved lava on the lower slopes of Kosthwaite Fell, and by the 

 garnet-rock of Cockley Beck in the Wrynose Valley. 



The cleaved garnet-lava of Kosthwaite Fell (3766) 

 shows the change very well. The garnet-fragments lose their 

 isotropic character at the margin, and yield high polarization-tints 

 between crossed nicols. The refractive index, however, remains the 

 same. There seems to be no intermediate stage between this and 

 the production of the colourless mineral, which has a refractive 

 index lower than that of the chlorite. Cleavage-lines are often well 

 shown, but the flakes do not always give straight extinction, the 

 angle of extinction being sometimes as great as 15° or 18°. The 

 mineral is exceedingly like a colourless mica, but might be either talc 

 or kaolin. If the mineral be a white mica, it seems almost impossible 

 to discuss the change chemically. The greater part of the iron of 

 the garnet is eliminated and the iron-ore set free, while the chlorite 

 would account for the rest of the iron, the magnesium, and possibly 

 the calcium ; but it is difficult to account for the alkali necessary 

 for the production of the colourless mica. There would be no great 

 difficulty in accounting for the production of either talc or kaolin. 



Garnetiferous lava and breccia are well developed on the small 

 plateau around Dock Tarn. A lava of considerable thickness over- 

 lies the intrusive sill, and has a silica-percentage of 64-2. 1 The 

 same rock occurs on the other side of the valley on the lower slopes 

 of Rosthwaite Fell. Above it is found the garnet-breccia of Papelay 

 Crag. A sill of considerable thickness is intrusive into this breccia, 

 producing similar metamorphism to that of the Dock-Tarn intrusive 

 rock. Intrusive junctions are to be seen in a field south-east of 

 Stonethwaite Village, where some friend of the geologist had done 

 some extensive blasting, and also on the east side of the valley close 

 to a small peat-bog. Ward mapped this rock as a lava, and, to explain 

 the intrusive junction last mentioned, brought in a thin basic dyke 

 from an intrusive mass occurring close to the Watendlath path. This 

 intrusive sill is closely connected with the intrusions round the old 



1 Harker, ' Chemical Notes on Lake-District Bocks ' in the 'Naturalist' for 

 1899, p. 57. 



