SOME ANCIENT AND MODERN VOLCANIC ROCKS. 405 



Summary. 



1. Specimens of trap collected from the Arans, the Arenigs, and 

 Snowdon and its neighbourhood, all have the same microscopic struc- 

 ture. 



2. This structure presents a hazy or milky-looking base with 

 scattered particles of a light-green or brownish dichroic mineral 

 (chlorite or its allies), and generally some porphyritically imbedded 

 felspar crystals, or fragments of such, both orthoclase and plagioclase 

 (probably oligoclase). In polarized light, on crossing the Nicols, the 

 base breaks up into an irregular coloured breccia, the colours changing 

 to their complementaries on rotating either of the prisms. 



3. Finely bedded ash, when highly altered, is undistinguishable 

 in microscopic structure from an undoubted felstone -lava. 



4. Ash of a coarser nature, when highly altered, is also very gene- 

 rally not to be distinguished microscopically from felstone, though 

 now and then the outlines of some of the fragments will reveal its 

 true nature. Metamorphism has sometimes given rise to a kind of 

 viscous flow of chloritic matter round the larger fragments. 



5. The fragments which make up the coarser ash-rocks seem 

 generally to consist of felstone containing both orthoclase and pla- 

 gioclase crystals or fragments ; but occasionally there occur pieces 

 of a more crystalline nature, with minute acicular prisms and pla- 

 gioclase felspar. 



6. In many cases the only tests that can be applied to distinguish 

 between highly altered ash-rock and felstone are, the presence of 

 a bedded or fragmentary appearance on weathered surfaces, and the 

 gradual passage into less-altered and unmistakable ash. 



IV. Examples oe Microscopic Rock-structttre among tee Lavas 

 and Ashes oe Cumberland. 



The following notes will afford only such a preliminary sketch of 

 these rocks as will suffice for a general comparison with those just 

 treated of. All details, and most of the conclusions arrived at from 

 a study of the Cumberland volcanic series in the field, during several 

 past years, have been necessarily reserved for the Survey Memoir 

 upon 101 S.E., now in the press. The Volcanic Series of Borrowdale 

 (green slates and porphyries) consists of alternations of contempora- 

 neous traps (lava-beds), ash, and breccia, with a thickness of many 

 thousand feet, and without any intercalations of ordinary sedimentary 

 material except quite at the base. Only within the last two years 

 has the microscopic examination of these rocks led me to recognize 

 any decided difference between the lavas of Cumberland and those of 

 Wales. I had, however, previous to the use of the microscope, sepa- 

 rated a well-marked series of undoubtedly contemporaneous traps, 

 from another great series of felstone-like rocks which, from evidence 

 in the field, I was led to believe were only highly altered ash-beds. 

 Microscopic study, I think, goes a long way towards confirming this 

 conclusion. 



