June 30, 1887] 



NATURE 



213 



theses, says Dr. Barrois, the conditions remind us somewhat of 

 the Dimetian proposed by Dr. Hicks for Wales. It is evident, 

 however, that he is more disposed to favour either of the other 

 hypotheses. 



Certain mica schists are largely developed in the southern 

 plateau ; they alternate with subordinate beds of fine-grained 

 gneiss, amphibolites, chlorite schists, micaceous schists, and 

 interstratified masses of diorites and "gneissites" of eruptive 

 origin. These accessory rocks form, together with the mica 

 schists into which they are injected, long parallel bands from 

 one end to the other of the southern plateau. The "gneissites " 

 include a complete assemblage of acid rocks, remarkable for 

 their gneissic, ribboned, and glandular structure, rich in white 

 mica, and in secondary feldspar, with "granulitic" quartz in 

 elongated grains, in rounded drops, and in thin flakes. These 

 include the rothe-gneiss, augen-gneiss, flaser-gneiss, Stengel- 

 gneiss, halleflintas, and leptynites of the German geologists ; as 

 also the rhyolitic felsites, volcanic breccias, halleflintas, and 

 felsitic tuffs of the English geologists. 



Is the injection of the "gneissites" contemporary with the 

 mica schists, or should it be referred to a subsequent epoch ? 

 If we accept the first of these hypotheses, the stage so termed 

 may be said to ally itself by its lithological characters to the 

 Arvonian of Dr. Hicks. It would seem, however, that in 

 making the (geological) sheets of Lorient and Chateaulin, Dr. 

 Barrois and his coadjutors have adopted another view, by 

 referring the characters of the >" gneissites " or "granulites 

 feuilletees " to their consolidation in special " encaissements " 

 under suitable mechanical conditions of depth and pressure, and 

 at an epoch different, but as yet undetermined, from that of the 

 massive granulite of Pontivy. 



The schists of Groix constitute a stage of micaceous schists, 

 of chloritic and chloritoid schists, of carbonaceous schists, and 

 of mica schists, especially remarkable for the abundance of the 

 heavy minerals which they contain (staurolite, garnet, magnetite, 

 &c. ). The carbonaceous or graphitic schists sometimes referred 

 to the Cambrian would appear to form the base of this stage. The 

 boundary between it and the Cambrian is admitted to be obscure. 

 If this stage, says Dr. Barrois, corresponds to the Pebidian of 

 Wales, it is distinguished by its poverty in interstratified basic 

 rocks, which are always of limited extent in Brittany. 



Cambrian. — How far these greenish-gray satiny schists, with 

 their beds of quartzite and veins of quartz, correspond to any 

 British Cambrian beds, the author admits is uncertain. More- 

 over, we would observe that there is no mention here of any bed 

 of conglomerate at the base of the Cambrian. Hence the 

 evidence as to the antiquity of the presumably pre-Cambrian 

 rocks fails in this important particular. The author estimates 

 this stage (Phyllades of Douarnenez) at over 3000 metres. 

 Above these are beds of schist and conglomerate in regular 

 interstratification. The conglomerates are formed of little 

 pebbles of quartz with about i per cent, of other stones. They 

 are distinguished from the Silurian conglomerates by the small- 

 ness of their component parts, and by their inferior hardness. 

 The equivalents of these beds in the north of the department are 

 fossiliferous, and correspond to the Paradoxides beds of la Vega 

 in the Asturias,^ and to stage C of Barraode. Here then we have 

 our first palaeontological horizon in Brittany which would seem 

 to be Menevian. 



Silurian. — The lowest sta .< ■ ;hus classified consists of red 

 schists, variegated quartzites, and beds of quartzose conglomerate. 

 This is succeeded by the famous " Gres Armoricain," which 

 forms the most salient feature of Menez-Hom and the Black 

 Mountains. It is characterised by Scolithes, Bilobites, Lingulce, 

 &c., and is the most constant of the fossiliferous beds of Finistere. 

 Barrande's stage D is represented by the slates of Angers with 

 Calymene tristani, &c. The three horizons of the third 

 Silurian fauna are with difficulty traced on the north of the 

 Black Mountains. 



Devonian. — From a geognostic point of view the schists and 

 quartzites of Plougastel, over 1000 metres in thickness, con- 

 stitute the most important stage of this system, being largely 

 developed in the roadstead of Brest and forming the northern 

 crest of the Black Mountains. Homalonotus sp. , Rhynchonella 

 puilloni, and Grammysia davidsoni, are amongst the few 

 fossils. Above these come beds recalling the Taunusian, 

 Coblenzian, and Eifelian, for the most part fairly fossiliferous. 

 As no higher ones are mentioned, we may presume that the 

 Middle and Upper Devonian are absent. 



,' See Geological Magazine, i£8j, p. 274. 



Carboniferous. — The physical history of Brittany during this 

 period was one of oscillation between terrestrial and marine con- 

 ditions ; it was a period of extensive eruptions and of great earth- 

 movements. Hence a considerable portion of the sediments, 

 especially towards the base, are of volcanic origin. The mass 

 of the formation is comprised in what Dr. Barrois calls the 

 "schists of Chateaulin," an alternation of schists, slates, and 

 sandstones with Spirifer striatus, Slrophomena rhomboidalis, 

 Phillipsia derbyensis, and Prodiictus semireticulalus : they also 

 contain poor impressions of plants. In some respects this 

 description reminds us of the Culm of Devonshire. This group 

 rests unconformably on the various Devonian beds. The actual 

 Coal-Measures form three small and distinct basins in Finistere 

 of little economic value. 



It is interesting to note that the volcanic phenomena in this 

 region are referred to the Carboniferous rather than to the 

 Devonian epoch, and this serves to recall the controversy as to 

 the precise geological age of the rocks in the Brent Tor district 

 — a doubt which is applicable to a large area of Palaeozoic rocks 

 lying to the north-west of Dartmoor. Since, in Brittany, the 

 Carboniferous rocks are unconformable to the Devonian, whilst 

 the intermediate deposits consist in many places of porphyritic 

 tuffs, it is evident that the chief deposit of ashes and other 

 volcanic material represent formations intermediate in respect of 

 time. Why may they not in part be Middle and Upper 

 Devonian ? To the Carboniferous period also are referred the 

 porphyroid granites of Rostrenan and other places, and the 

 numerous veins of quartz porphyry, which are so apt to 

 follow the synclinal folds of the sedimentary rocks, the prevailing 

 direction being a little to the north of east. The eruptions, 

 according to Dr. Barrois, must have commenced after the 

 Devonian, and continued during the whole of the Lower Carboni- 

 ferous. The most important development of quartz diorite, 

 which follows the southern foot of the Black Mountains, he 

 regards as posterior to the Devonian and anterior to the 

 Carboniferous. 



Lastly, Dr. Barrois speculates on the earth-movements that 

 have helped to fashion the country of Finistere, which may be 

 said to possess a radiate structure in consequence of the numerous 

 flexures undergone by the rocks ; the general orientation is east 

 to west, but with a tendency to converge towards west. These 

 directions correspond to axes of a complete series of synclinals 

 and anticlinals. The eruptive rocks of the region have been 

 affected at the same time as the sedimentary rocks, whose fold- 

 ings they have followed ; they made their appearance chiefly 

 at two epochs, during the Archasan (terrain primitif) and during 

 the Carboniferous, thus affording two periods of maximum 

 eruptive force. The principal periods of flexing appear to have 

 been five in number, and correspond in the main to the breaks 

 in the great systems already detailed. The fifth and greatest 

 flexure took place after the Upper Coal-Measures : it has left its 

 mark on all the formations, and since that period Finistere has 

 been in a condition oi terra firtna. W. H. H. 



TEMPERATURE IN RELATION TO FISH. 

 T^HE influence of temperature exerts itself to such a marked 

 degree upon the habits, food, reproduction, and migra- 

 tion of fish, that observations upon the subject are essential in 

 determining the relations of certain forms to their surroundings. 

 The National Fish-Culture Association have for some time past 

 made investigations into the temperature of the ocean, not 

 only at the surface, but also at the bottom, and the Council 

 will shortly publish the results. In order to ascertain its 

 effect upon fish maintained under artificial conditions, Mr. 

 W. August Carter, of that body, has compiled the following 

 statistics, showing the influence of temperature upon fish at 

 the late South Kensington Aquarium, where the average 

 depth of the tanks was 4 J feet. The statistics are derived 

 from observations made daily during a period of three years 

 by noting the temperature of the water in the tanks, and 

 the death-rate prevalent at certain seasons of the year. By 

 observing the degrees of temperature at which certain fish 

 succumbed from time to time, Mr. Carter has drawn an 

 average, showing the temperature adapted to various fish, and 

 their capacity, in some instances, for withstanding extremes of 

 heat and cold. 



It must be borne in mind that the temperatures recorded are 

 applicable only to fish in confinement, and living therefore under 



