Mr* J. C. Ward on Ancient and Modern Volcanic Rocks. 329 



gioclase crystals or fragments ; but occasionally there occur pieces 

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

 gioclasc 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 unmista- 

 kable 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 tabulse 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 

 greystone 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 composition they are neither true basalts nor true felstones. 

 In petrological structure they have much the general character of 

 the modern Vesuvian lavas — the separate flows being usually of no 

 great thickness, 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 different from that of 

 such old basalts as those of South Stafford and some of those of Car- 

 boniferous 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 tra- 

 chytes, these old Cumbrian rocks might perhaps be called Felsi- 

 dolcrites, answering in position to the modern Trachy-dolcrites. 

 | A detailed examination of Cumbrian ash-rocks had convinced the 

 author that in many cases most intense metamorphism had taken 

 place, that the finer ashy material had been partially melted down, 

 and a kind of streaky flow caused around the larger fragments. 

 There was every stage of transition from an ash-rock in which a bedded 

 or fragmentary structure was clearly visible, to an exceedingly close 

 and flinty felstone-like rock, undistinguishable in hand specimens 



