64 A DESCRIPTION OF THE [Ch. V. 



The granite and the mica-slate not only occur in each other 

 in the state of irregular portions, but in numerous instances 

 were found, by Mr. Weaver, to alternate in large beds, the 

 position of which is so distinct, that their nature cannot admit 

 of a doubt, as in the case of Polgooth mine, in Cornwall. This 

 phenomenon is so important in the history of primary rocks 

 that it is desirable to quote some examples in detail. 



" In Glenmalur, nearly one half of Lugduff, which borders 

 the glen on its north side, is composed of these alternations, 

 the study of which has been greatly facilitated both by nature 

 and art ; the northern face of Glenmalur being furrowed by 

 several ravines, and the lead mine of Ballinafinchogue being 

 situated in this quarter. Ascending the ravine, the water of 

 which turns the wheel of the upper smelting-house, we find 

 the following succession of beds : 



1. Granite, at bottom, of uncertain thickness, 



upon which rests 



2. Mica-slate, including many layers of granite, 



which alternate with the mica- slate, 



forming a body - 24 feet thick. 



3. Granite - - - 12 



4. Mica-slate and granite, forming an indis- 



tinct intermixture - - - 36 



N. B. The preceding beds include also fre- 

 quent thin layers of quartz. 



5. Granite, the lower part of which is most cu- 



riously interwoven with contempora- 

 neous veins of granite and quartz - 90 



6. Mica-slate - 462 



7. Repeated alternations of granite and mica- 



slate, forming a body - - - 132 



8. Mica-slate - - 372 



9. Granite - 9 



10. Mica-slate - 120 



11. Granite - - - - 30 



