2 12 



NATURE 



[June 30, 1887 



the Dryas leaves was undoubtedly obtained. Some samples of 

 this tufa show a relatively rapid precipitation, the needles and 

 pieces of bark themselves being sometimes found intact on the 

 cleaving of the tufa. In this layer are also found remains of a 

 species of broad-bladed grass, and Herr Carlson has further 

 found in it the imprint of a feather. Uppermost, at all events 

 in certain spots, mosses only are found. The calcareous tufa, 

 the greatest thickness of which is hardly more than 3 feet, is in 

 turn covered with peat. 



In the tufa are sometimes found, in layers, thin bands of re- 

 mains of plants, chiefly of grasses and mosses, the vegetable 

 substance of which is still preserved. In such a layer even a 

 leaf of Betula nana was found. Besides the above-mentioned 

 remains of plants may be mentioned leaves of at least three 

 different kinds of Salices, one reminding oi S. cinerea, one of 

 S. repens, and one which most certainly cannot be referred to 

 any of the varieties now found in Southern Sweden. In ad- 

 dition to those of Betula nana, imperfect leaves of a large birch- 

 tree, probably B. odora'a, have been found, and also a perfect 

 leaf of one apparently corresponding with B. intermedia, al- 

 though there is some probability that it may be a smaller leaf of 

 B. odorata, it being generally impossible to define leaves varying 

 so much as those of Betula and Salix from a single imprint. 



Of the species just named, two at all events, viz. Betula nana 

 and Dryas octopetala, are extinct near Vadstena. It is also pro- 

 bable that one of the varieties of Salix is now foreign to these 

 parts. Betula nana, however, is still found in certain parts of 

 Ostergotland, but the nearest spot in which Dryas grows is in 

 the mountains around the valley Herjedalen, about 4° to the 

 north-west of the lake Wettern, and we may safely assume that 

 the presence of these two plants in the same locality clearly 

 indicates that at the time of the deposition of the older layers of 

 calcareous tufa the climate was much colder than that now pre- 

 vailing there. For my own part, I am even disposed to consider 

 this discovery of fossil Dryas near Wettern as a proof of a purely 

 Arctic flora having prevailed in these parts at an age older than 

 that represented by the calcareous tufa ; studies of the same in 

 the province of Jemtland in my opinion indicating that a true 

 Arctic climate is not favourable to the development of calcareous 

 tufa, this mineral being first deposited after the climate has 

 become milder. In Jemtland we certainly find Arctic plants 

 in the tufa, but generally together with remains of pine, and 

 they must therefore be considered as the last remnants of an 

 Arctic flora, which already then was in course of being sup- 

 planted by the pine and accompanying species. Its greatest 

 significance lies, not only in the proof of an Arctic flora once 

 having flourished in these parts, but also in the circumstance 

 that it proves that an Arctic flora could exist at such a low 

 elevation. 



It will further appear from the above exposition that the 

 Arctic flora in this locality was followed by a pine vegetation, 

 the process thus entirely corresponding with what took place in 

 Scania and Norrland. Information from other localities in 

 these parts is, however, required before we can arrive at general 

 conclusions. 



Finally, it may not be out of place here briefly to refer to a 

 question which to some extent may be considered to be affected 

 by this discovery. In a paper read in i860 before the Academy 

 by Prof. Sven Loven, "On some Crustacea found in the lakes 

 Wettern and Wenern," the author pointed out that, as re- 

 gards Wettern, this lake sheltered a fauna belonging to deeper 

 waters, of originally marine and at the same time Arctic 

 character. This fauna Prof. Loven considered to be a relic 

 from the time when Wettern, by way of the Baltic and Lake 

 Ladoga, was connected with the Arctic Ocean. He said :— 

 " Some few favoured species, those which in a higher degree 

 than others were able to adapt themselves to the new medium, 

 and which already in their former habitat, the less saline Arctic 

 Ocean, had accustomed themselves to live for instance where 

 melting glaciers diluted the sea- water, or at the mouth of rivers, 

 would in one or another of the great lakes thrive longer than 

 others, and finally be the only ones surviving. Such a lake is 

 Wettern." It should be pointed out that the discovery of the 

 fossil Dryas leaves on the shore of the lake Wettern is of con- 

 siderable significance in view of the opinion thus expressed by 

 Prof Loven. For the calcareous tufa referred to here having 

 been deposited since the sea had already receded from these parts, 

 and this tufa nevertheless containing Arctic plants, we may con- 

 clude that the lake Wettern became separated from the sea 

 whilst the climate was still Arctic. A. G. Nathorst. 



GEOLOGICAL STRUCTURE OF FINISTERE. 

 'P'HIS article is founded upon the " Aper9u sur la constitution 

 geologique du Finistere," prepared for a recent excursion 

 of the Geological Society of France by Dr. Charles Barrois, of 

 Lille. 



Since the three great promontories of South Wales, Devon 

 and Cornwall, and Brittany, are sharers in no small degree of a 

 common geological history, English geologists can hardly fail to 

 take an interest in the structure of the western extremity of 

 Brittany. Dr. Barrois is very well known to many of us, and 

 the fact that for some time past he has been engaged on the 

 geological survey of Brittany renders his observations all the 

 more valuable. From time to time he has furnished the annals 

 of the Societe Geologique du Nord with some of the results 

 of his observations in that country. Of these we may mention 

 "Le granite de Rostrenan (Cotes-du-Nord), ses apophyses et ses 

 contacts," " Memoire sur les schists metamorphiques de I'ile 

 de Groix (Morbihan)," "Memoire sur les gres metamorphiques 

 du massif granitique du Guemene (Morbihan)," " Note sur la 

 structure stratigraphique des Montagnes de Menez (C6tes-du- 

 Nord)," and " Legende de la feuille de Chateaulin (Finistere)." 



The department of Finistere is traversed from east to west by 

 two parallel chains — on the south the Black Mountains, on the 

 north the Mountains of Arree. Between the first-named chain 

 and the Atlantic lies the southern plateau of Brittany, whilst 

 the northern plateau is situated between the Mountains of Arree 

 and the English Channel. Both plateaux are formed by 

 Archaean (primitifs) and Cambrian rocks more or less injected 

 by granite. The basin included between the two ranges presents 

 a series of beds extending from the Silurian to the Carboniferous 

 in parallel folds, and is evidently one of the most important 

 physical features in the north-west of France. 



The stratified rocks of the region present the following 

 succession : — 



Carboniferous. 



Schists and Conglomerates of the Coal-Measures. 

 Schists and Sandstones of Chateaulin. 

 Porphyritic Tuffs. 

 Conglomerates and Porphyritic Tuffs. 



Devonian. 



Nodular Schists of Porsgruen. 

 Schists and Limestones of Nehou. 

 White Grit of Landevennec. 

 Schists and Quartzites of Plougastel. 

 Limestone of Rosan with S. looiensis. 



Silurian. 



Nodular Schists with C. interrupta. 



Bituminous Schists with Graptolites. 



White Sandstones. 



Slates of Angers. ^ 



"Gres Armoricain." 



Conglomerates and Red Schists of La Chevre. 



Cambrian. 



Schists and Conglomerates of Gourin. 

 "Phyllades" of Douarnenez. 



Archaan. 

 Schists of Groix. 

 Mica Schists of Audierne. 

 Granitic Gneisses of Pont-Scorff. 



Archaan. — The most ancient group of rocks in Finistere 

 consists of certain granitic gneisses and mica schists. The 

 gneisses are devoid of white mica, consisting mainly of white 

 and rose feldspar in large grains, with abundance of black mica, 

 in foliations, sometimes replaced by hornblende in fragments, 

 with granitoid and secondary quartz. These gneisses alternate 

 with interstratified beds of mica schists and amphibolites, and 

 pass into gneissic granites which penetrate them after the 

 manner of an eruptive rock. 



The injection of this gneissic granite may be explained in 

 three different ways: (i) either it is contemporary with the 

 gneiss and the mica schists, or (2) it may date from a later 

 epoch, or (3) lastly, it may proceed directly from the gneisses by 

 means of local Tecrystallisations under the action of a powerful 

 general metamorphism. If we accept the first of these hypo- 



