372 PROF. T. G. BONNEY ON THE GAENET-ACTINOLITE [Aug. 1 898, 



certain of them are rather fissile, thej are, as a general rule, fairly 

 well consolidated : the minerals of the groiindmass presenting a 

 mosaic structure, usually without any obvious indication of crushing. 

 The same remark holds good, so far as I have examined them, of 

 the other gneisses and schists (including the granitoid mass called 

 the Fibbia Gneiss) on the upper part of the St. Gothard Pass ; but 

 the gneisses on the northern side of the great trough occupied by 

 the head-waters of the Reuss and Rhone exhibit much more distinct 

 signs of having been crushed. Parts of the felspars have indeed been 

 replaced by minute white mica and free quartz, but the reconstruc- 

 tion here is less complete than in the case of the Tremola Schists. 

 Again, while the general trend of the great rock-masses at the 

 St. Gothard is not h\T from west to east, the strike of the apparent 

 bedding and the cleavage-foliation in the Tremola Schists generally 

 varies from N.E. to N.N.E., a strike indeed which, as it has been 

 more than once observed, is sometimes very marked, and can be 

 detected at intervals, in almost every part of the Alpine chain. This 

 structure, for reasons already published, I regard as pre-Triassic. 

 Hence I think it probable that the Tremola Schists attained very 

 nearly to their present condition at a date prior to the mountain- 

 making of the existiug Alpine system. 



Discussion. 



Gen. McMahon remarked on the important character of this 

 communication. The Author had added to the list of rocks originally 

 regarded as sedimentary but now proved to be of igneous origin. 

 The influence of fluxion-movements on an igneous rock prior to 

 consolidation, especially when more or less magmatic differentiation, 

 or the injection of one rock by another igneous rock, had taken 

 place, was receiving more and more recognition. This action had 

 recently received a beautiful illustration in the experiments of 

 Mr. H. S. Hele-Shaw : — colouring-matter injected into flowing 

 water curving round obstacles, and imitating in a remarkable way 

 the foliation of gneissic and banded rocks. The Author had also 

 proved in his paper that the structure now to be seen in rocks was 

 sometimes the result of complex causes : the production of secondary 

 mica being subsequently superinduced by pressure on a rock whose 

 main structural features were due to causes operating prior to 

 consolidation. Garnets were one cf the roost common products of 

 contact-metamorphism, but they were also present in many igneous 

 rocks as original constituents, as shown by their sometimes con- 

 taining such igneous minerals as beryl ; he therefore had no doubt 

 that the Author was correct in regarding, as he understood him to 

 do, the garnets in the rock described as original minerals. 



Prof. Watts was glad to hear it conceded that the bulk of the 

 Tremola Schists were originally igneous rocks. In reference to a 

 remark made bj- the previous speaker, he wished to observe that 

 while the earlier advocates of dynamo-metamorphism may have 

 been wrong in attributing too much to pressure, all recent papers 



