Atignst 5, iSSo] 



AT A TURE 



327 



local increase of temperature within certain subterranean 

 horizons. But, as he himself admits, this way of putting 

 the case brings us no nearer to what may be the ultimate 

 cause of such a local increase of temperature. He seeks 

 to prove that all the phenomena of volcanic action point 

 to local excitation, and that the observed order of appear- 

 ance of lavas is what on this view might theoretically be 

 anticipated. It would be beyond the necessary limits of 

 this article to follow him into the details of his argument. 

 But one or two points may be briefly referred to. He 

 regards la\-as as mainly derived not from primeval sub- 

 terranean magmas, but rather from the fusion of such 

 rocks as the crystalline schists and sedimentary forma- 

 tions. In the mechanics of eruptions he believes that the 

 outpouring of lava does not arise from the expansion of 

 vapours absorbed within the molten magma, but is merely 

 a hydrostatic problem of the simplest order — the lava 

 being forced out by the weight of the rocks overlying its 

 subterranean reservoirs. 



According to this hypothesis two preliminary conditions 

 are requisite for an eruption of lava — the rocks must be 

 fused, and their density in the molten state must be less 

 than that of the o\'erlying rocks. The author regards the 

 observed order of appearance of the lavas to be deter- 

 mined by their relative density and fusibility, the more 

 siliceous requiring a higher temperature to fuse them, and 

 the more basic, though less refractor)-, demanding a 

 higher temperature to give them such a diminution of 

 density as will permit them to be "erupted. At an early 

 stage of eruption he holds that the acid rocks may be 

 light enough to be ejected, but are not yet melted, while 

 the basic rocks may be melted but must await further ex- 

 pansion by access.of heat before they are capable of being 

 poured forth. Hence some intermediate rock will be 

 selected as the first to issue, and this rock the author 

 belie\-es to be propylite. A further increase of tempera- 

 ture produces hornblendic andesite and trachyte, and so 

 on to the rhyolites, and finally the basalts. All rocks 

 more basic than propylite are stated to present evidence 

 of superfusion, these rocks, according to the theory, being 

 those which possess so high a density as to demand a 

 much greater accession of heat than that required for 

 mere fusion, in order that they may become lighter than 

 the overlying crust, and thus be erupted. Basalt in 

 particular is cited as an example of a superfused rock. 



The author tacitly assumes that the density of a lava 

 at the time of its outflow is necessarily less than that of 

 the rocks through which it ascends, otherwise it could 

 not be erupted. It is a pity that no experimental demon- 

 stration of this assertion was given, for it forms so funda- 

 mental a postulate in the hypothesis. But even on the 

 supposition that the lava is forced out by the descent of 

 heavier overlying rock, what ought to be found as proof 

 of this action.' Ought we not to meet with abundant 

 evidence of subsidence at volcanic foci ? Every mass of 

 lava derived from the local fusion of rocks at no great 

 depth beneath the surface and driven out by the weight 

 of roik overlying it, should have an accompanying and 

 proportionate subsidence of the crust over the site of 

 its source. Occasional proofs of collapse at volcanoes 

 have long been known indeed, but admit of other 

 explanation, such as " evisceration," to use Mr. Mallet's 

 phrase. Instead of subsidence, the emission of vol- 

 canic material has generally been accompanied with 

 upheaval. Capt. Dutton's own magnificent Plateaux of 

 Utah should furniih copious proofs of a sinking or 

 sagging of the nearly horizontal strata round the eruptive 

 vents. But there is no trace of any structure of this 

 kind in his instructive and carefully-drawn sections. 



Again, the alleged superfusion of the basic rocks can 

 hardly be admitted upon the evidence here brought for- 

 ward in its support. The fact that thin streams of basalt 

 have had a greater liquidity and have retained it for much 

 greater distances than the acid lavas, has long been 



recognised. But as Reyer has recently suggested, it is 

 capable of a different interpretation from that of super- 

 fusion. The author appeals also to the microscopic 

 structure of basalt as favouring his view of former intense 

 ignition. He cites, for e.xample, the presence of glass 

 particles, the absence of water-cavities, the isotropic base, 

 the compactness and vitreous structure of this rock. But 

 are not these characters present in far more striking 

 development among the vitreous acid rocks, which he 

 supposes to have had a temperature little more than 

 sufficient for fusion ? The exceptions which the author 

 candidly admits to occur in the normal succession of 

 lavas — basalts, for example, appearing before rhyolites, or 

 quartz propylite and quartz-andesite simultaneously with 

 the hornblendic members of their respective groups— seem 

 fatal to the hypothesis. 



From another point of view the idea that the order of 

 emission of lavas has been determined in the way sup- 

 posed presents great difficulties. The author affirms that 

 " we must at least admit that the source of lavas is 

 among segregated masses of heterogenous materials," 

 and he supposes that " this arrangement would be well 

 satisfied by a succession of metamorphic strata [gneiss, 

 hornblendic and augitic schist] resting upon a supposed 

 primitive crust or magma having a constitution approxi- 

 mating that of the basaltic group of rocks." But every 

 known mass of metamorphic strata presents endless inter- 

 stratifications of very various materials. By what process 

 of selection are the elements of these diverse rocks grouped 

 succe;si\'ely into definite volcanic compounds ? How is 

 it that out of the simmering subterranean broth just so 

 much silica and alumina as are needed for one tj'pe are 

 ladled out at one time, while a careful hand is kept on 

 the lime, alkalies and iron-oxides, only the right propor- 

 tions being dealt forth for each lava ? 



The remarkable persistence of type among the different 

 species of lava all over the world has long been recog- 

 nised. It is not easy to see how this persistence should 

 exist, nor why there should not be far more varieties of 

 lava and transitional grades between the varieties if they 

 are due to the local melting up of various masses of 

 heterogeneous materials within the crust. 



The volume is illustrated by'a^series of heliotype plates, 

 from photographs taken in the course of the survey, 

 representing some of the more remarkable external forms 

 assumed by the sedimentary and volcanic rocks. The 

 Atlas contains a valuable series of topographical and 

 geological maps. Among these a relief-map of the 

 Plateaux, on the scale of five miles to an inch, is 

 specially instructive. There are likewise two plates of 

 sections, which bring before the eye in a clear and con- 

 cise form the structural details of the region. In point 

 of execution the plates of the atlas are altogether ad- 

 mirable. In his preface Capt. Dutton states that he 

 undertook the task of exploration assigned to him with 

 considerable diffidence in his ability to accomplish it. 

 He must be congratulated on having achieved a signal 

 succers. His work bears everywhere marks of the most 

 conscientious and painstaking industry, great acuteness 

 of observation, and not a little literary skill in the mar- 

 shalling and presentation of the facts observed. Let us 

 hope that the arrangement by which he was enabled to 

 exchange the routine duties of an army officer for geolo- 

 logical field-work may be prolonged, and that in further 

 prosecution of his explorations in the West he may live 

 to issue other volumes as interesting and valuable as that 

 which is noticed here. ARCH. Geikie 



TWO NEW PLANETARY NEBULA 



A PLANETARY nebula in R.A. iSh. 2S'2m. and 

 Dec. — 25° 13' was discovered at the Harvard 

 College Obser\'atory on the evening of July 13. A second 



