Vol. 50.] GNEISSES IN THE C0TTIAN SEQUENCE. 259 



ward, traversing the band of ' central gneiss.' That geologist has 

 described the sections along tbis line with much care. Hence it 

 was natural that I should expect to discover much closer agreement 

 with tbe facts than in regions to the north, where the lines must 

 have been hastily and diagrammatically sketched in. I therefore 

 turned to the valley of the Po in the hope of finding the gneiss 

 exactly where Zaccagna has placed it in bis map. In this I was not 

 disappointed : the gueiss can be seen in numerous roadside exposures 

 between the hamlet of Calcinere, west of Paesana, and the western 

 end of a gorge due south of Crampetti. The gneiss forms steep, 

 barren, craggy slopes, easily distinguishable from the softer and 

 more fertile schists. 



The section which best shows the relations of the gneiss and the 

 schists occurs on the south bank of the river near a footbridge, at 

 the upper end of a gorge east of Crissolo. The base of the hillside 

 here is of coarse augen-gneiss, which is covered by the schists. The 

 normal schist at more than 20 feet from the gneiss is a thin-bedded 

 lead-coloured rock, with some intorstratified amphibolites. 



As we approach the gneiss the schists become coarser, till they 

 form a gneissoid mica-schist — such as is generally observed near the 

 contact with the Waldensian gneisses throughout the Cottians. 

 The lead-coloured schists contain much material that can be iden- 

 tified as having been originally clastic grains ; as we approach the 

 gneiss this lessens in amount and water-clear quartz-felspar mosaic 

 becomes the chief constituent. This is associated with numerous 

 garnets, and some muscovite, epidote, and chlorite ; there is also 

 some indeterminate material which represents the less altered part 

 of the original rock. Still nearer the gneiss the garnets disappear, 

 but some earthy grej r zoisite-aggregates represent the original 

 constituents of the ' pietre verdi ' ; the bulk of the rock is formed of 

 white mica, orthoclase, and quartz-orthoclase mosaic. 



In addition to this contact-metamorphism, further proof of the 

 intrusive nature of the gneiss in this section is afforded by some 

 dykes of aplite which strike off from the gneiss ; these are best seen 

 where they cross some bands of amphibolites which occur in the 

 schists. Owing to the great difference in the chemical composition 

 of the aplite and tbe amphibolites, the junction between them is 

 much sharper than that between the aplite and the mica-schists ; 

 it is very irregular, sending small projections into the amphibolite 

 in a way that clearly shows the intrusive nature of the former. 

 Unfortunately, it was not possible to trace the aplite-dyke into the 

 gneiss, as the junction was hidden by moraine matter, and it was 

 not even possible to be absolutely certain that the amphibolites 

 were in situ. Their occurrence as blocks along a definite narrow 

 line, however, rendered it highly probable, and any doubt was re- 

 moved by the discovery at the foot of the slope of a large boulder of 

 mica-schist containing a vein of amphibolite. 



There is, moreover, a marked discordance between the strike of 

 the gneiss and that of the schists, which, in the absence of any 

 evidence for a fault, affords further proof of intrusion. 



