ISO 



KNOWLEDGE. 



[August 2, 1897. 



genius of such men as Sir Archibald Geikie ; and their 

 testimony has been supplemented in such wise that the 

 position of long vanished seas, rivers, and lakes may 

 be ascertained. " Popular imagination has long been 

 prone to see signs of volcanic action in the more prominent 

 rocky features of landscape. A bold crag, a deep and 

 precipitous ravine, a chasm in the side of a mountain, have 

 been imhesitatingly set down as proof of volcanic dis- 

 turbance. Many a cauldron-shaped recess, like the corries 

 of Scotland or the cwms of Wales, has been cited as 



Fig. 2. — Flow Structure in the Lowest Felsite on the Track from 

 Llanberis to the Top of Snowdon. 



an actual crater, with its encircling walls still standing 

 complete " ; but these are misconceptions which vanish in 

 the light furnished by the relics of the ancient volcanoes of 

 Great Britain. 



There is no reason to believe that modern volcanoes 

 differ in any essential respect from those of past ages in 

 the earth's history, and it is possible, therefore, by studying 

 the materials thi-own up by the former and comparing 

 them with the ejectamenta and other remains left behind 

 by the latter, to obtain conclusive proofs of the existence 

 of once active volcanoes in this country. In the British 

 Islands ancient volcanoes of the Vesuvian type are well 

 represented. Such a volcano when in activity might 

 appear as outlined in Fig. 1. By prolonged denudation 

 the cone would eventually be worn down, the crater dis- 

 appearing ; and the only relic of the eruptive orifice may 

 be the central lava column (p) sohdified within the vent. 

 The waste continuing, it might in course of time reach the 

 original surface of the ground on which the volcano built 

 up its heap of ejected material, the central lava plug 

 being left as an isolated eminence, marking the site of the 

 former vent, as if to commemorate some tragic historical 

 event. 



Although all outward traces of actual volcanoes may 

 have been effaced, the erupted materials left in the 

 neighbourhood possess certain characters which dis- 

 tinguish them from other constituents of the terrestrial 

 crust ; thus the extrusive rocks are of two kinds — (1) the 

 lavas poured out in a molten condition at the surface, 

 and (2) the fragmental materials, including all kinds of 

 pyroclastic di'triuis discharged from volcanic vents. One 

 can weU imderstaud that in such rocks, chemically formed 

 by the reaction of different materials as in a gallipot, the 

 composition will differ in a marked degree from sedimen- 

 tary rocks, many of which are to a great extent formed by 

 the mechanical mixture of diverse materials. Lavas, 

 however, furnish more easily appreciated characteristics. 

 For example, such a structure of rock undoubtedly took 

 its origin in molten masses from the expansion of 



imprisoned gases, something after the fashion of yeast in 

 the process of fermentation, and thus forms a crucial test 

 in deciding the once liquid condition of rocks which dis- 

 play it. The vesicles are sometimes flattened owing to 

 elongation of the steamholes by movement of the lava 

 before its consolidation, and by the trend and grouping of 

 these elongated cavities the probable direction of the flow 

 of the lava before it finally came to rest may be deter- 

 mined. The presence of glass, or some result of the 

 devitrification of an original glass, is so abundantly found 

 in lavas that it may safely be inferred that the original 

 condition of most lavas was vitreous. " Not only are 

 vestiges of the original glass recognizable, but the whole 

 progress of devitrification may be followed into a crystal- 

 line structure. The primitive crystallites or microlites 

 of different minerals may be seen to have grouped them- 

 selves together into more or less perfect crystals, while 

 scattered crystals of earlier consolidation have been 

 partially dissolved in and corroded by the molten glass. 

 These and other characteristics of once fused rocks have 

 to a considerable extent been imitated artificially by MM. 

 Fouque and Michel Levy, who have fused the constituent 

 minerals in the proper proportions." 



One of the most striking features is the frequency and 

 remarkable flow structure represented in Fig. 2. By 

 making a sufficient number of observations of the direction 

 of this flow and projecting backwards, the point of inter- 

 section of all the lines of flow would probably indicate 

 the site of the original vent. " I would," says Sir 

 Archibald, "refer to one of the basement felsites of 

 Snowdon, which forms a line of picturesque crags on the 

 slope facing Llanberis. The layers of variously devitrified 

 matter curl and fold over each other, and have been rolled 

 into balls, or have been broken up and enclosed within 

 the other. The general push indicated by them points to 

 the westward. Turning round from the crags, and 

 looking towards the west, we see before us on the other 

 side of the deep vale of Llyn Cwellyn, at a distance of 

 little more than three miles, the great dome-shaped 

 Mynydd-mawr, which, there is every reason to believe, 

 marks one of the orifices of eruption. It might in this 

 way be practicable to obtain information regarding even 



Fio. 3. — Ejected Block of Basalt which has fallen among Carboni- 

 ferous Shales and Limestones. Shore, Pettyeur, Fife. 



some of the vents that still lie deeply buried under 

 volcanic or sedimentary rocks." 



Among the volcanic records of former periods, one 

 occasionally comes upon a single block of lava embedded 

 among tuffs or even in non-volcanic strata. We know 

 that by the tremendous explosions of volcanoes at the 

 present day such stones are often deflected from the 

 vertical and projected to immense distances from the vent, 

 and there can be little doubt that the blocks of basalt 

 (Fig. 8) found from time to time, were, in the distant past, 

 hurled like thunderbolts from the now extinct volcanoes 

 in our own peaceful island. To those who desire to know 

 more of this fascinating subject, we would say, " Read the 



