, SOME ANCIENT AND MODERN VOLCANIC ROCKS. 421 



Mr. Porbes remarked that too much stress should not be laid 

 upon the apparent order in which the constituents in a lava had 

 crystallized or solidified, as it had been experimentally proved that 

 this order was not necessarily that of their fusing-points, and cited, 

 as an example to be seen every day on the large scale, the Pattinson 

 process for desilverizing lead, in which the lead is seen to solidify in 

 crystals at a temperature at which the infinitely less fusible silver 

 remains fluid. As a rule he had not found any difficulty in distin- 

 guishing, under the microscope, between a normal lava and one 

 which had become reconsolidated from its ash or tuff, and protested 

 against the very frequent application of the term Yolcanic Ash to 

 beds which were true volcanic tuffs. In answer to the question as 

 to what was the difference between ashes and tuffs, he denned ashes 

 as purely subaerial formations, thrown out of the volcanic orifice, 

 and falling down on to land or sea. as the case happened ; whilst 

 tuffs, on the contrary, were molten lava poured out into or more 

 often under water, and thus instantaneously quenched and disin- 

 tegrated into fragments or powder more or less fine in proportion as 

 the action of the water was overpowering. In ashes each separate 

 particle bore on its exterior the evidence of its having been exposed 

 to the action of fire in the throat of the volcano, and externally is 

 altered, glazed, or coated with a crust or skin, often resembling that 

 of a meteorite, an appearance which is never to be observed in tuffs. 



Prof. Ramsay said that he thought Mr. "Ward was on the right 

 track, and that the microscopic study of rocks must lead to most 

 valuable results. He rejoiced to find that the determination of the 

 nature of the rocks of the Welsh district, made by the Geological 

 Survey some thirty years ago, was now confirmed by microscopical 

 research. This seemed to show that the larger features of a district 

 might be worked out without the aid of a microscope. He stated 

 that his conclusion was that the ashes had been thrown out of Old 

 Silurian volcanoes, first beneath the surface of the sea, and after- 

 wards above water as the vents increased in height. In the Cum- 

 brian area, after the very earliest eruptions there, the volcanoes 

 were subaerial, and greenslates were fine ashes thrown out upon 

 land. 



Mr. Bonnet stated that he had recently examined the " Green- 

 Slates-and-Porphyries " series, and wished to bear testimony to the 

 value of Mr. Ward's researches, and the great difficulty of deter- 

 mining some of the metamorphosed rocks. There was, however, no 

 doubt that the latter, notwithstanding the extent to which, through 

 metamorphism, they simulated the appearance of igneous rocks, were 

 originally stratified volcanic ashes or tuffs. He had not yet been 

 able to examine his own specimens microscopically ; but so far as he 

 could determine without that, he should class some as diabase, 

 others as "porphyrite." He called attention to the remarkable 

 lithological correspondence between the " Green-Slate " series of the 

 Lake District and the so-called Cambrian of Charnwood Porest, the 

 volcanic breccias, ashes or tuffs, and hornstones of both being often 

 very similar. It was remarkable that the strike of these " Green 



