Sept. 15, 188 1 J 



NATURE 



471 



SECTION C— Geology 



On the Laiirentian Beds of Donegal and of other Parts of Ire- 

 land, by Prof. Echvard Hull, LL.D., F.K.S., &c., Director of 

 the Geological Survey of Ireland. — After a perusal of the 

 writings of previous authors, and a personal examination made 

 in the spring of 1881, in company with two of his colleagues of 

 the Geological Survey, Mr. k. G. Symes, F.G. S., and Mr. S. 

 B. Wilkinson, the author had arrived at the following conclu- 

 sions: — I. That the Gneissose series of Ponegal, sometimes 

 called " Ponegal granite," is unconformably overlaid by the 

 metamorphosed quartzites, schists, and limestones which I'rof. 

 Harliness had shown to be the representatives of the Lower 

 Silurian beds of Scotland (Quart. Jmnn. Ceol. See., vol. xvii, 

 p. 256). 1 his unconformity is especially noticeable in the dis- 

 trict of Lough Salt near Glen. 2. That the Gneissose series is 

 similar in character and identical in position and age with the 

 " Fundamental Gneiss" (Murchison) of parts of Sutherlandshire 

 and Ross -shire, and is therefore, lil>e the latter, presumably of 

 Laurentian age. That the formation is a metamorphosed series 

 of sedimentary bed^, has been shown by Lr. Haughton and Mr. 

 R. H, Scott. 3. That the noi th-w estern bcundary of the 

 Donegal gneiss is a large fault between the laurentian gneiss 

 and the metamorphosed Lower Silurian beds, owing to which 

 the older rocks have been elevated, and by dei udation have 

 been exposed at the surface, 4. That the Cambrian formation 

 of Scotland is not represented in I'onegal, and that the uncon- 

 forniily abve referred to represents a double hiatus, ard is of the 

 same character as that which occurs in Sutherlandshire, in the 

 district of Fornaven and Ben Arkle, where the Lower Silurian 

 beds rest directly on the Laurentian gneiss. 5. That Laurentian 

 rocks may be recogniied in other parts of Ireland, as in the 

 .Slieve Gamph and Ox Mountains of Mayo and Sligo, at Pel- 

 mullet, and in West Galway, north of Galway Bay, where the 

 recks consist of red gneiss, horneblende rock, and schist, &c., 

 similar to those in L'onegal ; also possibly in Co, Tyrone, as 

 suggested by Mr. Kinahcn. 



Laurentian J^ocks in Ireland, by G. H. Kinahan, M.R.I. A., 

 &c. — Ihe writer first ir.entioned that Cainozoic and Mesozoic 

 rocks only occurred in the province of Ulster, while in the rest 

 of the island there was a nearly continue us sequence of Pala;ozoic 

 rocks, proved by the work of Griffith, Jukes, and their subordi- 

 nates, from the Coal-Measures down to the Cambrian. He then 

 pointed out that a recent attempt had been made to try and dis- 

 turb their natural order, but that the new theory was sclely 

 fcunded on assertions that would not bear investigation. He 

 proceeded to observe that the geologists of the pre-Cambrian 

 school appeared to lay more weight on lithological evidence than 

 that to w hich it w as entitled, and in continuation he gave the 

 localities for the oldest rocks in Ireland, with the reasons for and 

 against the rocks being Laurentian. 1 he localities are Carngore, 

 or South-East Wexford, while it was shown that althcugh the 

 rocks were lilhologically similar to the Laurentian?, yet they 

 contained Cambrian fossils^ Ca/it'o;', South- East Mayo, Sligo, 

 and Leitrim — rocks that, from their lithological character', were 

 said to be I aurentian by Murchison, who recanted his statement 

 when Harkness showed that stratigraphically this was an im- 

 possibility. These rocks occur on two zone';, those on the 

 highest being now- said to be Laurentian — Erris, Norlh-West 

 Mayo — very eld recks, about which nothing can be positively 

 said, except that they are older than the associated metamorphic 

 rocks, also of uncertain age. Donegal, Lciulcnderry, and 

 Tyrone — the Laurentian age of some of these, years ago, was 

 suggested by Jukes, while now it is positively asserted, 1 ut solely 

 on hthological characters. The author pointed eut that, althcugh 

 lithologically very like Laurentians, they were more like Hu- 

 ronians, Logan's description of the latter being very suitalle for 

 those of Donegal. He also pointed out that it was tmnecessary 

 to make vague asrertions, as the stratigraphical position of the 

 rocks ought to be easily worked tut, either by starting from the 

 Pomeroy fossiliferous rocks, or from the fossilifercus rocks 

 found in Donegal 1-y Dr. King ; 1 ut that, at the same tin.e, the 

 work n.ust be much better and more correctly done than that in 

 the neighbourhood of Pomert y, w here the unaltered fos' iliferous 

 beds are classed with those they lie on, although the latter were 

 extensively metamorphosed, contorted, upturned, and denuded, 

 prior to the'fossiUfereus rocks being deposited on them. North- 

 East Antrim-XGokf, supposed to be of the same age as the older 

 rocks near Pom»eroy (Offer Cambrians). 



On the Occurrenee of Granite tn situ about Twenty Miles 

 Scuth-Wtst of Eddystone, by A. R. Hunt, M.A., F.G.S.— The 



author described and exhibited a fragment of granite brought up 

 by a Brixham trawler twenty miles south-west of the I ddystone. 

 He believed it to have been torn off a mass of granite in situ, 

 and pointed out that in mineral com.position it agreed w ith the 

 gneisses of the Eddystone Keef and of the Shovel Keef in 

 Plymouth Sound — all these recks being composed of mica, 

 quartz, and felspar, without hornblende or schorl. 1 he author 

 believed that the occurrence of gneiss in Plymouth Sound with- 

 out altering the adjacent Devonian rocks was an indication that 

 these Channel typical gneisses, and probably the typical granites 

 too, w ere of pre-Devonian age. 



'ome Obsenations on the Causes of Voleanic Action, by J. 

 Prestwich, M.A., F.R.S., &c , Professor of Geology in the 

 University of Oxford. — The hypothesis generally accepted 

 in this country as to the cruse of volcanic rction is that 

 of the late Mr. Poulett Scrope, who considered that "the 

 rise of lava in a volcanic vent is occasioned by the expan- 

 sion of volumes of high-pres-ure sleam, generated in a miass 

 of licjuefied and heated matter within or beneath the erup- 

 tive orifice," and that the expulsion of the lava is effected 

 solely by high-presure steam generated at great depth.s, 

 but at what depths is not mentioned, nor is it explained 

 how the water is introduced, whether from the surface, or 

 whether from water in original combination with the basic 

 magma. The objections to this hypothesis are— I. That during 

 the most powerful explosions, i.e., when the discharge of steam 

 is at its maximum, the escape of lava i^ frequently at its minimum. 

 2. That streams of lava often flow with little disengagement of 

 steam, and are geneially greatest after the fe>rce of the first 

 violent explosion is expended. 3. That it is not a mere toiling 

 over, in which case, after the escape of the active agent — the 

 water— and the expulsion of such portion of the obstructing 

 medium, the lava, as becam.e entangled with it, the remaining 

 lava would subside in the vent to a depth corresponding to the 

 quantity of lava ejected ; l;ut the level of the lava, eatiris paribsu, 

 remains the sam.e during successive eruptions. Of the important 

 part played by w ater in volcanic eruptions there can be no 

 d( ubt, but instead of considering it as the primary, the author 

 views it as secondary cau;e in volcanic eiuptions. All 

 agree in describing ordinary volcanic eruptions as generally 

 accc mpanied or preceded by shocks or earthquakes of a minor or 

 local character, to which succeed paroxysmal explosions, during 

 which vast quantities of -tones, scorix, and ashes, together with 

 vtlumes of steam, are prcjected from the crater. The first 

 paroxysms are the most violent, and they gradually decrease and 

 then cease altogether. T he flow of lava, on the other hand, w hich 

 commences sooner or later after the first explosions, is continued 

 and prolonged independently. Ultimately the vclcano returns 

 to a state of repose, which may last a few months or many years. 

 Adopting the theory of an original igneeus rucleu=, the .".uthor 

 considers a certain fluidity of the fom^.er, and mobility of the 

 latter. The one and the other feebly represent conditions of 

 which the phenomena of the rocks afford clearer ar.d stronger 

 evidence as we go back in geological time. Altheugh thermo- 

 metrical experiments, of the necessary accuracy and length of 

 time, are yet wanting, it has 1 een estimated that a smiall quantity 

 of central heat still reaches the surface ard is lost by radiation 

 into space, and the escape of liquid lava and steam from vol- 

 cances, and of hot sjirings from the^e and ether seurce?, n.ust 

 bring, in however small a quantity-, a certain increment of heat 

 from the interior to the surface, where it is lost. This sheuld 

 lead to a certain contraction at depths, and of readjustment of 

 the external crust, in consequence of which the fused masses of 

 the interior w ill from time to time tend to be forced outwards, 

 whenever tension becam.e sufficient to overcom.e resistance. 

 In this the author agrees with many other geologists. The 

 further hypothesis respecting volcanic action, he now ugge.sits, he 

 has keen n,ainly led to form by his researches on underground 

 waters. A portion of the rain falling on the surface not only of 

 permeable and fissured sedimentary strata, kut abo of fissured 

 and creviced crystaUine and other rocks, passes below ground, 

 and is there transmitted as far dow n as the pern: cable recks range, 

 or as the fiss ures in the rocks extend, unless some counteracting 

 causes intervene. Those causes are the occun'ence of imper- 

 meable recks, faults, and heat. The former tw o are exceptional, 

 the latter constant. The increase of temperature with depth 

 being l' Fahr. for every 50 to 60 feet, the boiling point of 

 water would be reached at a depth of abcut lc,coo feet, lut 

 owing to the pre.ssure of the superincumbent recks, it has been 

 estimated that water will retain its liquidity and continue 



