Vol. 50.] GNEISSES IN THE COTXIAN SEQUENCE. 235 



lowest mica-schists : they approach very close to the gneiss, but 

 never enter it. Gastaldi's map L of the Paradiso, on the contrary, 

 actually shows one broken across by the gneiss (fig. 2, p. 240). In reply 

 to the enquiry as to whether any case were known of an intrusion of 

 the ' pietre verdi ' series into the central gneiss, Prof. Sacco assured 

 me that such is impossible, as the occurrence of one of this series in 

 the gneiss would prove that this belongs to the newer gneisses. 

 Gastaldi, who fully recognized both the fact and its significance, 

 boldly claimed 2 that this proved that the ' pietre verdi ' rocks were 

 not igneous but bedded sediments : if they were intrusive, he argued, 

 they would certainly have somewhere cut the basal gneiss. There 

 seemed, however, another alternative, viz. that the central gneiss 

 may be newer and not older than the overlying schists ; Gastaldi 

 does not appear to have considered this explanation, which was not 

 opposed to any of the facts known to me. Prof. A. C. Lawson's 

 remarkable work on the geology of Rainy Lake, 3 proving that the 

 ' LaurentiaD ' gneiss was there intrusive into the overlying rocks 

 which have been correlated with the Huronian, showed the possi- 

 bility of the truth of this explanation. Prof. Lehmann 4 and Dr. 

 Danzig's 5 confirmation of Kallmann's view of the intrusive nature 

 and post-Archaean age of the Saxon gneisses and granulites, and Mr. 

 Barrow's 6 description of intrusive gneiss-dykes in Forfar, suggested 

 the possibility that the intrusive nature of the gneiss might not be 

 exceptional. Moreover, M. Michel-Levy's 7 demonstration that the 

 protogine gneiss which forms the nucleus of Mont Blanc is an 

 eruptive granite, and not a basal gneiss, showed that the same 

 relation holds in at least one place in the Alps. There are, how- 

 ever, considerable differences between the protogine gneiss of Mont 

 Blanc and the fresh gneisses of the Cottians, and as these mountain- 

 groups belong to different Alpine zones, which are arranged on very 

 different plans, I was not inclined hastily to apply conclusions from 

 one to the other. 



Impressed by these doubts, I was led to devote most of the time 

 at my disposal for field-work during the summer of 1892 to a care- 

 ful examination of the Cottian gneisses, in the hope that by working 

 along the junctions some sections might be found that would settle 

 the exact relations of the two series. At the same time it became 

 necessary to settle the nature of the relation between the schists and 



1 Gastaldi, Mem. descriz. Carta geol. Italia, vol. i. (1871) pi. vi. 



2 Gastaldi, in letter to T. S. Hunt, Geol. Mag. 1887, p. 536. 



3 A. C. Lawson, ' Eeport on the Geology of the Rainy Lake Region,' Ann. 

 Rep. Geol. Surv. Canada for 1887, pt. F, 182 pp and map. 



4 J. Lehm inn, ' Die Entstehung der altkrystallinischen Schiefergesteine, mifc 



besonderer Bezugnahme auf das sachsische Granulitgebirge ' Bonn, 2 vols., 



1884. 



5 E. Danzig, ' Ueber die eruptive Natur gewisser Gneisse sowie des Granulits 

 im sachsischen Mittelgebirge," Mitth. Min. Instit. Univ. Kiel, vol. i. (1888) 

 pp. 33-79. 



,; G. Barrow, ' On certain [Highland] Gneisses,' Geol. Mag. 1892, pp. 64-65 ; 

 [see also Quart. Journ. Geol. Soc. vol. xlix. (1893) pp. 330-356.] 



7 Michel-Jbevy, 'Etude sur les rocb.es cristallines et eruptives des environs du 

 Mont-Blanc,' Bull. Serv. Carte geol. France, no. ix. 1890, 26 pp. and plates. 



