HARDWICKE'S SCIENCE-GOSSIP. 



279 



prisms of the Solfatara rock. The order of crystal- 

 lization of the componeut minerals was shown to 

 be the following : — magnetite, felspar in large or 

 small distinct crystals, augite, felspathic or leucitic 

 solvent. Some of the first-formed crystals were 

 broken and rendered imperfect before the viscid 

 state of igneous fusion ceased. Even in such 

 modern lava-flows as that of the Solfatara consider- 

 able changes had taken place by alteration and the 

 replacement of one mineral by another, and is very 

 generally in successive layers corresponding to the 

 crystal outlines. The frequent circular arrange- 

 ment of the glass and stone-cavities near the cir- 

 cumference of the minute leucite crystals in the 

 lava of 1631 was thought to point to the fact 

 that after the other minerals had separated from the 

 leucitic solvent, the latter began to crystallize at 

 numerous adjacent points; and as these points 

 approached one another, solidification proceeded 

 more rapidly, and these cavities were more 

 generally imprisoned than at the earlier stages 

 of crystallization. In the example of the lava 

 of 1794, where the leucite crystals were further 

 apart, this peculiar arrangement of cavities was 

 almost unknown. The third part of the paper dealt 

 with the lavas and ashes of North Wales ; and the 

 author thought that the following points were 

 established: — 1. Specimens of lava from the Aran.s, the 

 Arenigs, and Snowdon audits neighbourhood, all Jiave 

 the same microscopic structure. 2. This structure 

 presents a hazy or niilky-lookiug base, with scattered 

 particles of alight-green dichroic mineral (chlorite)^ 

 and generally some porphyrilically imbedded felspar 

 crystals or fragments of such, both orthoclase and 

 plagioclase. In polarized light, on crossing the 

 Nicols, the base breaks up into an irregular-coloured 

 breccia, the colours changing to their complemen- 

 taries on rotating either of the prisms. 3. Einely- 

 bedded ash, when JiigJdy altered, is in some cases 

 undistinguishable in microscopic structure from 

 undoubted felstone. 4. Ash of a coarser natures 

 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 crystal- 

 line nature, with minute acicnlar prisms and pla- 

 gioclase i'elspar. G. 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 unmistakable 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 tabulai being often curved, and breaks 

 with a sharp conchoidal and flinty fracture. Silica 

 59-61 per cent. Matrix generally crystalline, con- 

 taining crystals of labradorite or oligoclase and 

 orthoclase, 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 thtit of such old basalts as those of 

 South Staflbrd 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 incliued to regard most of 

 them as occupying an intermediate place between 

 felsitic and doleritic lavas ; and as the felstone lavas 

 were once probably trachytes, these old Cumbrian 

 rocks might perhaps be called Eelsidolerites, 

 answering in position to the modern Trachy- 

 dolerites. 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 

 transition from an ash-rock in which a bedded or 

 fragmentary structure was clearly visible, to an 

 exceedingly close and flinty felstone-hke rock, 

 undistinguishable in hand specimens I'rom a true 

 contemporaneous trap. Such altered rocks were, 

 however, quite distinct in microscopic structure 

 from the undoubted lava-flows of the same district, 

 and often distinct also from the Welsh felstones, 

 although some were almost identical microscopically 

 with the highly altered ashes of AYales, and together 

 with them resembled the felstone lavas of the same 



