670 Reports and Proceedings — 



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 higlily altered, is 

 in some cases nndistinguishable in microscopic structure from un- 

 doubted felstone. 4. Ash of a coarser nature, when highly altered, 

 is also very frequently not to be distinguished from felstone, though 

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

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

 seem generally to consist of felstone, containing both orthoclase and 

 plagioclase crystals or fragments ; but occasionally there occur pieces 

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

 clase felspar. 6. In many cases the only tests that can be applied 

 to distinguish between highly-altered ash-rock and a felstone are 

 the presence of a bedded or fragmentary appearance on weathered 

 surfaces, and the gradual passage into less altered and unmistak- 

 able ash. 



In the fourth division of his paper the author described some of 

 the lavas and ashes of Cumberland of Lower Silurian age. 



With regard to these ancient lavas the following was given as a 

 general definition : — The rock is generally of some shade of blue 

 or dark-green, generally weathering white round the edges, but 

 to a very slight depth. It frequently assumes a tabular structure, 

 the tabulee being often curved, and breaks with a sharp conchoidal 

 and flinty fracture. Silica 59-61 per cent. Matrix generally crys- 

 talline, containing crystals of labradorite or oligoclase, and ortho- 

 clase, porphyritically imbedded, round which the small crystalline 

 needles seem frequently to have flowed; magnetite generally 

 abundant, and augite tolerably so, though usually changed into a 

 soft dark-green mineral ; apatite and perhaps olivine as occasional 

 constituents. Occasionally the crystalline base is partly obscured 

 and a felsitic structure takes its place. 



The Cumberland lavas were shown to resemble the Solfatara grey- 

 stone in the frequent flow of the crystalline base, and the modern 

 lavas generally in the order in which the various minerals crystallized 

 out. In external structure they have, for the most part, much more 

 of a felsitic than a basaltic appearance. In internal structure they 

 have considerable analogies with the basalts. In chemical compo- 

 sition they are neither true basalts nor true felstones. In petrolo- 

 gical structure they have much the general character of the modern 

 Vesuvian lavas ; the separate flows being usually of no great thick- 

 ness, being slaggy, vesicular, or brecciated at top and bottom, and 

 having often a considerable range, as if they had flowed in some 

 cases for several miles from their point of eruption. Their general 

 microscopic appearance is also very difierent from that of such old 

 basalts as those of South Stafford and some of those of Carboniferous 

 age in Scotland. 



On the whole, while believing that in some cases the lavas in 

 question were true basalts, the author was inclined to regard most 

 of them as occupying an intermediate place between felsitic 

 and doleritic lavas ; and as the felstone-lavas were once probably 



