572 Reports and Proceedings. 



examining sections of rocks by other tests than that of mere microscopic scrutiny 

 prior to the final mounting of the preparations. He was inclined to regard some 

 of the ashes, of which sections were exhibited, as volcanic ejectamenta, imbedded 

 in a base much resembling devitrified, or partially devitrified matter allied to 

 pitchstone ; and he could not agree with the author in regarding the fluxed 

 character of this base as a structure superinduced by metamorphism, at all events 

 not in the restricted sense of that term. 



Mr. Koch thought that the flow of materials around crystals may be due to 

 secondary actions. The effects of tension applied to molten matter may produce 

 appearances similar to those described by the author. The study of the optical 

 properties of crystals produced under tension would, he considered, materially 

 assist in the determination of the nature of the crystals in rock-masses. He 

 regarded chemical analysis as not furnishing a sufficient test. 



Mr. Forbes 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's 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 distinguishing 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 Volcanic Ash 

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

 the difference between ashes and tuffs, he defined ashes as purely subaerial forma- 

 tions, 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 disintegrated 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 ex- 

 ternally 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 micro- 

 scopical 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 volcanos, first beneath 

 the surface of the sea, and afterwards above water as the vents increased in height. 

 The green-slates were fine ashes thrown out upon land. 



Mr. Bonney 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 gi-eat difficulty of determining 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 detei-mine 

 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 

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

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

 last seen to the S.E., was about W.N.W., which corresponded nearly with the 

 Charnwood strike. Hence he thought it possible that they might be of the same age. 



Mr. Judd remarked that the circumstances connected with the mode of occur- 

 rence of certain very important rock-constituents, such as leucite, haliyne, nosean, 

 mellilite, etc., seemed to point to the conclusion that species of minerals may 

 present peculiarities of distribution, both in space and time, analogous to those of 

 animal and vegetable species. He suggested the necessity of constantly keeping 

 in view such a possibility when making comparisons between ancient and modem 

 volcanic rocks. 



