344 Dr. A. P. Young — Structure of the Tarntal Mass. 



already veined with quartz, appears to have split into parallel- walled 

 plates, between which the other rock was injected, though probably 

 not in its present form. 



A microscope section of the schist shows it to consist in great 

 part of fine fibres of minerals of the mica, chlorite, and serpentine 

 groups, with probably some talc, the whole woven into a felt much 

 resembling the more fibrous specimens of the Tarntal serpentine. 

 It contains also clusters of small grains of a carbonate, derived no 

 doubt from the adjoining dolomite ; scattered grains of quartz and 

 orthoclase, generally without strain, shreds of a colourless mica with 

 small axial angle (2 E = 57° nearly ; near sericite), and a few zircons, 

 some ilmenite and rutile, all of which might be derived from a gneissic 

 rock ; also tourmaline in minute prisms, in some cases large enough to 

 show the interference figure of a negative uniaxal crystal in trans- 

 verse sections with polygonal outline, and the higher absorption of 

 the ordinary ray in longitudinal sections. 



The schist thus represents one of the earlier intrusions of the 

 igneous magma, the foliation being the effect of later dynamic and 

 chemical processes which have not appreciably altered the structure 

 of the dolomite. 



This solution of the problem would make the earliest intrusion of 

 the basic magma younger than the Tarntal dolomite, that is, later than 

 Trias if, in accordance with generally accepted views, this breccia is 

 to be correlated with beds of later Triassic age. The alternative 

 assumption, viz. Tarntal breccia older than Trias, is not excluded by 

 known facts and seems to be worth considering. 



As already mentioned, the dolomite knolls of the Knappenkuchel 

 have been referred to the Carboniferous on the ground of their 

 resemblance to the ' Eisendolomit ' of the Nosslacher Joch, where the 

 rock is associated with grauwackes bearing impressions of Zithodendron, 

 Annularia, Sphenophyllum, and other Carboniferous plants.' The 

 Tarntal dolomite has occasionally the reddish tint of the Nosslach rock. 

 All the rocks named contain some quartz. 



The alternation of dolomite and schist in the Mieselkopf occurrence 

 is too regular to have been brought about by violent movements of 

 folding and crushing, and is probably for the main part an oecogenous 

 character ; the foliation of the schist may be due in part to mechanical 

 causes, and is therefore possibly to some extent an apcecous character. 



The talc inclusions in the fragment of Tarntal breccia already 

 mentioned as having been found in the Upper Tarntal can be best 

 explained as due to an injection of fluid magma such as can be seen in 

 schists near the contact with serpentine. 



The appearances indicate that the Tarntal breccia was deposited 

 directly on the schists, after these rocks had acquired their foliated 

 character to some extent. It is no longer feasible to treat the breccia 

 as a * Nappe ' separate and distinct from the schists of the Reckner 

 block. The Tarntal dolomite belonged from the first to the transported 

 block, and was carried with it in the movement which landed the rocks 

 in their present position. It follows that the Eeckner block of 



^ Kerner, " Der Carbouflora des Steinaclierjoches " : Jahrhueh der Reichsanstalt, 

 TFien, 1897. 



