September 22, 1904] 



NA TURE 



507 



eruptions has been practically nioditied into that of 

 eruption from a large number of small and easily con- 

 cealed vents, which were doubtless situated along 

 fissures; and this theory is borne out by observed 

 occurrences along rifts on Etna. Mr. Marker himself 

 points out (p. 14) that " the great thickness and extent 

 of the basalt group results only from the superposition 

 and overlapping of a vast number of separate flows, 

 each ol which is of very insignificant dimensions." 



The small group of trachytes and rhyolites (p. 56) 

 in Skye occurs clearly between two series of basalts, 

 a fact very satisfactory for Irish geologists, who have 

 had to maintain similar views for their acid lavas on 

 less convincing evidence. Much of the interest of 

 the pelrographic details furnished by Mr. Marker lies 

 in the attack of one rock on another. The dissection 

 of an early peridotite by an overpowering mass of 

 gabbro (p. 64), the occurrence of " xenoliths " of one 

 kind of gabbro in another (p. 121), the mutual modifi- 

 cations of gabbro and granite in the Red Mills (p. 183), 

 which are justly attributed to a process of diffusion, 

 may be named as examples of the important problems 

 dealt with. We confess to a personal interest in the 

 results of Mr. Marker's researches 

 on composite sills (p. 209), where 

 an acid rock is shown to pick up 

 fragments of an earlier basic one, 

 and to discharge, as it w^ere, its 

 own porphyritic crystals into the 

 latter along its margins. Such 

 facts make us douljtful of the 

 necessity' for assuming a distinction 

 between " segregation-veins," with 

 ill-defined edges, and " ordinary 

 veins of intrusion " (p. 78), or for 

 the belief that bands, the crystals 

 of which interlock at the common 

 surface, " must have existed side 

 by side in the fluid state " (p. 119). 

 The discussion of the banded 

 gabbros, with the beautiful plates 

 accompanying it, forms a chapter 

 of exceptional value, though it 

 hardly does justice to Mr. Marker's 

 own recent work, in which he has 

 explained the genesis of a Cain- 

 ozoic banded gneiss. The refer- 

 ence (p. 115) of the " pyroxene- 

 granulites " of Skye to " altered 

 representatives of basic lavas en- 

 tangled in the gabbro comple.x " is in happy agree- 

 ment with the most modern views as to similar rock- 

 masses in the Saxon metamorphic area. 



.\ local rock of hybrid origin is termed marscoite 

 (]•>]•>. 175 and 192); this occurs as sills, and is regarded 

 as a basic magma modified by the absorption of 

 granitic material prior to its intrusion. Mr. Marker 

 holds that such " hybrid rocks are essentially abnormal 

 in composition " ; yet it may be urged that in the 

 deeper plutonic regions many rocks, which we have 

 come to consider normal, have arisen by processes o' 

 admixture and diffusion. The junction of the granite 

 and the Cambrian limestone in Skye (p. 135) presents 

 evidence of solution of the limestone, without addition 

 of lime to the granitic magma. We prefer to believe 

 that a slow diffusion and transference of the lime 

 occurred into the great subterranean mass, or that the 

 locally modified granitic magma flowed on elsewhere, 

 leaving new and unmodified material in contact with 

 the limestone, r;ither than to conclude that a rock 

 which absorbed gabbro and Torridon Sandstone be- 

 haved in a mysteriously different manner towards 

 dolomitic limestone. The singular absence of veins 

 passing from the granite into the limestone rather 



NO. 1 82 I, VOL. 70I 



suggests rapidity of solution. But the case is 

 certainly an uncommon one, as other contacts of these 

 two rocks show. Down l>elow us as we write, an 

 ancient granite sends off zig-zag veins abundantly into 

 the " Dalradian " limestone of Donegal; behind us, 

 the same granite produces a coarse quartz-diorite, by 

 interaction with a basic sill of the same series. A 

 little south, at Sunimy Lough, the pre-Devonian 

 gabbros have brought up inclusions of an earlier 

 gabbro, which " weather into little hollows," like those 

 of the corrie of Tairneilear. In the universality of the 

 problems discussed by Mr. Marker lies their wide 

 geological attraction ; and we venture to think that 

 many of his questions will receive their answer in more 

 distinctively plutonic regions. For a long time. Con- 

 tinental geologists maintained that some fundamental 

 difference separated our modern lavas from the crvstal- 

 line masses revealed in older regions of the crust. 

 Similarly, our fluidal gneisses, with their mutual inter- 

 actions, have been held to be something primordial 

 and apart. This purely mental barrier is now rapidly 

 breaking down, and we may find that the phenomena 

 so carefully set before us in the case of Skve have 



Rocks of Skye 



deep-seated and more impressive representatives in the 

 floor of Saxony or Norway. 



Mr. Marker's book, with its handsome photographic 

 illustrations, is published at a very moderate price, 

 seeincr that it appeals entirely to the professed geologist. 

 Indeed, when dealing with so superb an area, we think 

 that a little more descriptive power might have been 

 used to unite the scenic and the petrographic features. 

 We thus wish that chapter xxvi., on " physical features- 

 and scenery," had sent ofT intrusive sheets into those 

 that went before it, and had even wrapped round much 

 of their contents as literary "xenoliths." With the 

 manner of the text we have little fault to find. It is 

 always clear and direct — far clearer, in fact, than the 

 explanatory diagram on p. 433. The Americans have 

 given us many worse names than " mugearite "; and 

 other authors beside Mr. Marker have written " amyg- 

 dule " in place of the obvious " amygdale." " Pheno- 

 cryst," like " cab " and " bus," must probably be 

 accepted as a compromise, though we expect better 

 things of scientific men ; and the incorrect use of 

 " granophyre," introduced by Rosenbusch, has become 

 widely tolerated through repeated publication. The 

 term " ophitic p!at"s," rather than " ophitic crystals " 



