Vol. 50.] GNEISSES IN THE COTTIAN SEQUENCE. 275 



4. The Gneissic Structure. 



This is shown not to be due to dynamo-metamorphism, but 

 to be an original fluxion- structure formed owing to the intru- 

 sion of the rock in a viscid condition. It is, therefore, not to 

 be expected that these fluxion-gneisses will produce such 

 extensive contact-metamorphism as rocks intruded at a higher 

 temperature. Other gneisses in the Cottians were formed by 

 the dynamo-metamorphism of igneous rocks and mechanical 

 sediments. The example of those writers who use the term 

 ' gneiss ' only in a structural sense is, therefore, followed ; and 

 the three groups met with in the Cottians are referred to as 

 clastic gneisses, metapyrigen -gneisses, and fluxion-gneisses. 



5. The Age of the Waldensian Gneisses. 



(a) They are later than the schists, and the uppermost member 

 of this series belongs in all probability to the Palaeozoic ; 

 this conclusion is based on : 



i. The stratigraphical evidence relied upon by Lory and 



Kilian ; 

 ii. The evidence of the radiolarian fauna in the calc- 



schists ; 

 iii. The schists, however, are unquestionably pre-Triassic. 

 (o) The gneisses have not been affected by the great Miocene 

 (post-Aquitanian, pre-Langhian, post-Helvetian, and pre- 

 Tortonian) earth-movements which have crushed and con- 

 torted the schists. 

 (c) The elevation of the marine Pliocenes to the height of 1500 

 feet shows that powerful earth-movements occurred in late 

 Pliocene times ; the intrusion of the Waldensian gneiss was 

 probably contemporary, and perhaps the cause of these. 



EXPLANATION OF PLATE XV. 



42 

 Fig. 1. Typical Waldensian Gneiss. From quarries at Bussoleno. X ^t- 



The minerals shown are large, eroded, dusty crystals of orthoclase 

 (o), water-clear quartz-orthoclase mosaic (x), numerous crystals of 

 muscovite (/«), and granules of kyanite (k). 

 Fig. 2. (a) Gneiss from the margin of the Paradiso massif, consisting of a 

 fine-grained, quartz-orthoclase mosaic (x), muscovite (m), included 

 granules of epidote (e) and aggregates of zoisite (j) and culoritic 

 material (c). 

 (b) An included fragment from the adjoining ' pietre-verdi ' series, 

 showing the indefinite groundmass, quartz, and glaucophane (gl). 



42 



From the Vonzo Valley, near Chialamberto. X 01 . 



Fig. 3. Junction of the aplite-dykes with the rocks of the 'pietre-verdi' series; 

 from the Kocce Ruccaglie, Angrogna Valley; see fig. 9, p. 257. 

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