42 FRANK DA IVSON ADAMS 



that of the granite is insignificant, so that while, even if it be 

 established that dioritic rocks may be produced by the solution 

 of limestone in granite, it would scarcely indicate this as a prob- 

 able origin for the great intrusions of these rocks met with in 

 various other places. 



The other class of granite contacts to be considered are those 

 in which the granite comes against slates, shales, and similar 

 argillaceous strata. In the " Livret Guide" prepared for those 

 taking part in the excursion, Lacroix states that there exist in the 

 Pyrenees, notably in the Haute-Garonne and Hautes-Pyrenees, 

 many granite contacts of the ordinary type, long since described, 

 which present the regular and usual succession of spotted slates, 

 spotted or nodular micaceous schists, and andalusite hornstones ; 

 but that the contact zones of the Haute-Ariege, visited on the 

 excursion, were chosen because they present striking examples of 

 a much more intense alteration, in character analogous to certain 

 other French occurrences described by Michel-Levy, which are 

 characterized by a marked " feldspathisation" of the invaded 

 rocks, indicating the "importance preponderate " of deep-seated 

 emanations in the case of igneous contacts, and affording evi- 

 dence of a wholesale incorporation of the invaded rock. 



"Au contact immediat du granite, en effet, s'observe une zone 

 constante dans laquelle les schistes et aussi les quartzites se 

 chargent de feldspaths, soit par imbibition, ces mineraux jouant 

 le meme role que le quartz dans les schistes micaces, soit par 

 injection en nature du granite lui-meme. II est possible de 

 suivre, pas a pas, tous les stades de feldspathisation et les pas- 

 sages insensibles entre ces schistes feldspathises (Leptynolites) 

 et le granite lui-meme." In certain cases, as in the valley of the 

 Baxouillade (near the Cirque de Ras), he goes on to say, these 

 two types of "feldspathisation" are superimposed in one and the 

 same rock, giving the invaded rock a gneissic facies, which in 

 certain cases is so pronounced that it resembles a veritable gneiss. 



This process of "feldspathisation" may thus, according to 

 Lacroix, take place in two ways, (i) by imbibition, and (2) by 

 the injection of little veins of granite all through the invaded rock. 



