March 15, 1888] 



NATURE 



459 



The neo-volcanic rocks, which are stated to be essen- 

 tially confined to Tertiary or post-Tertiary times, occur, 

 for the most part, as lava-streams and sheets, and are 

 often accompanied by tuffs. They are classified by Prof. 

 Rosenbusch as follows : — 



{a) Family of the Liparites and Pantellerites (equi- 

 valents, on the one hand, of the palit'o-volcanic quartz- 

 porphyries, on the other, of the granitic plutonic rocks). 



{b) Family of the Trachytes and basic Pantellerites 

 (equivalents of the palaeo-volcanic quartzless porphyries, 

 and of the plutonic syenites). 



{c) Family of the Phonolites and Leucitophyres 

 (equivalents of the plutonic elaeolite-syenites). 



{d and e) Family of the Dacites and Andesites 

 (equivalents of the porphyrites and diorites). 



\f) Family of the Basalts (equivalents of the melaphyres 

 and certain augite porphyrites in the palaeo-volcanic 

 series ; and of the gabbros and diabases among the 

 plutonic rocks). 



{g) Family of the Tephrites (equivalents of the 

 th2ralites, i.e. plagioclase-nepheline rocks of the plutonic 

 series). 



From this synopsis the merits of the new classification 

 may be appreciated. No classification that taxonomic 

 ingenuity may devise will wholly satisfy the desires of the 

 sanguine petrologist. Rocks, however much they may be 

 characterized by a certain amount of geological uni- 

 formity persistent over large areas (which have aptly 

 been termed " petrographical provinces"), are still, it 

 must be remembered, mere mineral aggregates ; and the 

 amount of possible variation, dependent on differences in 

 chemical constitution, and varying conditions of con- 

 solidation, is enormous. Rock-types, which may be 

 clearly defined and sharply separated on paper, will, in 

 the field, often be found passing over into one another by 

 gradations so imperceptible that the petrographer must 

 regard as hopeless any attempt to draw a hard and fast 

 line between them. 



A weak point in Prof. Rosenbusch's classification seems 

 to ;us his fundamental separation of the "dyke-rocks" 

 [Ganggesteine) from the plutonic and volcanic series 

 [Tiefcn- unci Ergussgesteine.) Both plutonic bosses and 

 volcanic sheets must necessarily be accompanied by 

 dykes or pipes through which the eruption took place, 

 and into the rocks composing which they pass by imper- 

 ceptible gradations. The author, indeed, calls attention 

 himself to this fact (on pp. 6 and 522), and proposes to 

 include under the head of " Ganggesteine " only those 

 rocks which occur solely in the form of dykes and are un- 

 accompanied by tuffs. Still, rocks so nearly allied as 

 these must necessarily be to the dykes and volcanic pipes 

 and necks in immediate connection with the centre of 

 eruption, should not, we think, be so widely separated 

 from them. On the other hand, we find placed in this 

 group rocks, such as granite-porphyry, which are known to 

 occur in boises, as, for instance, at Shap and at Dartmoor. 



As to the question of age, it is so far satisfactory that 

 the author has gone a step in what is surely the right 

 direction, in eliminating this factor from the consideration 

 ofthe plutonic rocks. With regard to the advisability of 

 retaining the separation into an older and a younger 

 series of the volcanic rocks. Prof. Rosenbusch refrains 

 frotn expressing an opinion (p. xi. of preface.) In con- 



nection with this question, we must draw attention to one 

 point. The structure characteristic of the dolerites 

 (diabases of the Germans) in which allotriomorphic 

 masses of augite are penetrated by idiomorphic crystals 

 and microlites of felspar, and which is known as ophitic 

 structure, occurs nowhere in more typical development 

 than in the dolerites of the Western Isles of Scotland 

 (described and figured by Judd) and of Iceland (Brdon), a 

 statement that anybody who has seen rock-sections from 

 these localities will support. Yet these rocks, apparently 

 because they are of Tertiary age, are placed by Prof. 

 Rosenbusch (pp. 725 and 733) with the basalts, and are 

 described as possessing ^^ intersertal structure" a struc- 

 ture characterized, according to the definition given on 

 p. 504, by the presence of a hypocrystalline interstitial 

 substance (mesostasis) wedged in between the felspars. 

 That some of the rocks in question contain small wedge- 

 shaped portions and films of glassy interstitial substance 

 nobody will deny ; but that many of them are perfectly 

 holocrystaUine and truly ophitic is equally beyond 

 question. 



Besides " intersertal structure " we notice two other 

 structural terms used now for the first time, viz. " pilo- 

 taxitic " and " hyalopilitic." The former is applied to a 

 holocrystaUine structure, especially characteristic of certain 

 porphyrites and basalts, in which the ground mass consists 

 essentially of slender laths and microlites of felspar in 

 felted aggregation, and often exhibits fluxion-phenomena. 

 The addition of films of glass produces ^^ hyalopilitic" 

 structure. 



New rock-names are Tholeiite (p. 504) and Alnoite 

 (p. 805). The former is given to a variety of augite-porphy- 

 rite with typical "intersertal structure." Certain North of 

 England dykes (the Hett dyke, Tynemouth dyke, and 

 Hebburn dyke) described by Teall, are referred to this 

 group. Several of the English, Scotch, and Irish traps, 

 described by Allport and Hull, are, according to the 

 author, olivine-tholeiites (p. 515). The word '■^Alnoite" is 

 applied by Prof Rosenbusch to a subdivision of the 

 melilite-rocks, hitherto classed with the melilite-basalts, 

 but differing from the latter by their occurrence in the 

 form of dykes and their near relation to the eleeolite- 

 syenites. 



Interesting to English readers are the remarks contained 

 on pp. 417, 418. In referring to the Cambrian quartz- 

 felsites and felsites of Wales, which have been described 

 by Messrs. Bonney, Cole, and Rutley, Prof Rosenbusch 

 compliments these authors on not having overlooked the 

 influence of dynamic metamorphism in developing their 

 present character. He then goes on to say that he has 

 been led, partly by Prof. Bonney's descriptions, partly by 

 the examination of sections, to the belief that two distinct 

 classes of rocks are here associated, viz. metamorphosed 

 eruptive rocks (schistose porphyries), and metamorphosed 

 slates and tuffs (porphyroides). A comparative study of 

 these rocks in connection with the " Lenne-porphyren '' 

 and the porphyroides of the Thuringer Wald would, the 

 author thinks, be productive of interesting results. Many 

 of these rocks (e.g. from between Llanberis and Cwm-y- 

 Glo, north-west of Cwm-y-Glo, Llyn Padarn, near 

 Llanberis ; also the nodular felsites from Conway Falls, 

 and the rock from Digoed) ought, judging from the 

 frequent occurrence of striated and microperthitic felspars* 



