116 /. R. Dakyns — Note on the Green and Skiddaw Slates. 



not. "Where several cracks existed, tlie green bands soon imited, 

 and the entire crystal was changed. In another instance the change 

 commenced at the surface and extended inwards, till only the 

 central portion was left unaltered ; or again, the change proceeded in 

 both ways simultaneously. 



The evidence afforded by the study of these sections is so com- 

 plete, that there cannot, I think, be a doubt as to the conclusion to 

 be drawn. The rock-specimen is from Hailstone Hill, and is a 

 well-crystallized, nearly black, basalt, of rather coarser grain than 

 the mass of the rock, and a " power " of one inch is sufficient 

 for its examination when cut into a thin section. 



One point which deserves more attention than it has hitherto 

 received, is the alteration which rocks have undergone subsequently 

 to their consolidation, it is one of the great difficulties the petrolo- 

 gist has to contend with, but it is not insuperable. The polariscope 

 will generally indicate the occurrence of metamorphism, and a 

 careful search for the least altered portions of a rock may enable 

 the enquirer to leam the history of the change, and thus gain a 

 more correct knowledge of the original rock. It is clear, for 

 example, that Olivine was one of the original constituents of the 

 basalt above described, for its pseudomorphs occur in every part 

 of the district in which the rock is found. 



I venture to hope that the insertion of these observations in your 

 valuable Journal may contribute in some slight degree to show 

 that a microscopical analysis may be advantageously employed when 

 a chemical analysis is impossible, and that the origin and structure 

 of many rocks and minerals will not be accurately known till the 

 microscope is far more extensively used than it hitherto has been by 

 Geologists and Mineralogists. 



VI. — Additional Note on the Unconformity between the 

 Green and Skiddaw Slates. 



By J. E. Dakyns, M.A., of tlie Geological Survey of England and "Wales. 



THE Unconformity between the Green and Skiddaw Slates, which 

 I pointed out in the last number of the Geological Maga- 

 zine, (see p. 56) as exhibited between Grange in Borrowdale and 

 Dale Head will, I think, explain satisfactorily the Geology of Der- 

 wentwater. 



If anyone refers to Euthven's map of the Lake Country, (edition of 

 1855,) he will see the division between the two sets of beds drawn 

 between Castlehead and Castlerigg, and thence in a south-west 

 direction on the north side of Eobinson. This is incorrect. Eobin- 

 son and Hindscarth are both composed of Skiddaw Slates, and 

 Castlehead, to the best of my belief, of Greenstone. The line I 

 drew runs from Lowdore by the bridge at Grange to Dale Head, 

 which is roughly parallel to Euthven's line, but nearly a mile and 

 a half distant from it to the south-east. 



Thus it will be seen that while the western side of Derwentwater 



