Yol. 50.] GNEISSES IN THE COTTIAN SEQUENCE. 263 



— the bulk of the intrusive rock and the nature of the beds with 

 which it has come in contact. Thus, as illustrations of the 

 former, we may refer to the very thin contact-zone around the 

 Angrogna dykes, which are apparently but tongues intrusive into 

 the schists, so that no continued flow of molten matter ever occurred 

 through them. The dyke at the upper Col de Vento is larger and 

 probably rose to a considerable height above the present level of 

 the ridge ; a wider zone has therefore been affected. Around the 

 gneiss massifs the contact-metamorphism is much more strikingly 

 developed ; here the extent varies according to whether the gneiss 

 meets the schists as a vertical wall, as it does in the gorge of the 

 Comba, or forms a low bank upon which the schists rest, as in the 

 valleys of the Po and the Dora and in the Paradiso massif on the 

 south side of the Valle Grande. In the latter cases the contact- 

 metamorphism is most strongly marked because the schists then 

 acted, to use the conventional illustration, as a ' piecrust ' ; they were 

 in contact with the gneiss over a wide area, and through them the 

 heat given off by the consolidating rock was slowly conducted. 



The second condition which determines the extent of the rneta- 

 morphism is the nature of the rock in contact with the gneiss. Where 

 this is one of the amphibolites or other rock of the ' pietre verdi ' 

 group, the junction is sharp and definite ; where the older rock is 

 of a comparatively simple composition, such as a quartzite, it may 

 be rendered more schistose, but few secondary minerals have been 

 developed in it. At places where the gneiss has been intruded into 

 a rock of a similar composition, such as mica-schist, a much greater 

 change has been effected ; the mica-schists have then been rendered 

 gneissoid, and it is often difficult in the field to determine the exact 

 line of separation between the two rocks. Thus, in the sections on 

 the south side of the Chisone, east of Meano, we were not at all 

 sure — in the field — of the exact position of the contact; the micro- 

 scope, however, at once enables the two to be separated. 



M. Michel-Levy has pointed out similar features in the contact- 

 phenomena of the gneiss of the Central Plateau of Prance. Thus 

 he shows ' that, where the gneiss (or foliated granite) occurs in great 

 mass and is intruded into rocks which are also acid in character, 

 they are united by a passage-zone combining the characters of the 

 two rocks. "When, on the other hand, the mass of intrusive rock 

 is comparatively small, as in a dyke, or where it cuts through rocks 

 of a very different chemical composition, then the contact is very 

 sharp. 2 



In concluding this section it may be remarked that the contact- 

 metamorphism affords additional evidence as to the conditions 

 under which the gneisses consolidated. Considering the massiveness 



1 Michel-Levy, ' Compte-rendu de la Course du 19 aout, de Semur a, Saulieu, 

 par la Motte-Ternant,' Bull. Soc. geol. France, ser. 3, vol. vii. (1879) pp. 852, 853. 



2 Id. 'Compte-rendu de la Course du 19 aout, d'Avallon a Chastellux, 

 of. clt. p. 845; 'Compte-rendu ... a AUigny, Goie, Pensieres,' ibid. p. 872 

 and ' Compte-rendu . . . de Semur a Saulieu . . ., ibid. pp. 85o, 8.34. 



