EXCURSION TO THE PYRENEES 4 I 



the granite. The conditions here are especially favorable for 

 such a process of solution, as the limestone is broken up into a 

 number of disconnected masses, thus exposing a large surface to 

 the action of the granite. 



No investigations into the chemical relations of the "Roche 

 Dioritique" have been made, or at least none have been pub- 

 lished ; its exact chemical composition is unknown. This, how- 

 ever, is a subject on which information is especially required. It 

 is most important to know in how far the composition of the 

 "Roche Dioritique" approaches that of the normal granite of the 

 district, with the addition of varying amounts of lime. It would 

 seem that processes of differentiation must in any case have been 

 at work in the mass after the solution had taken place, for it 

 seems difficult to understand how any addition of lime to a 

 granite magma can produce a peridotite. The only alter- 

 native, if this view of the origin of the mass is to be sustained, 

 is to suppose that the exhalations given off by the magma first 

 impregnated the invaded limestone in some places with minerals 

 rich in iron, elsewhere with those rich in magnesia, and having 

 thus produced out of the limestone a series of rocks of very 

 diverse composition, the magma bodily dissolved them and thus 

 obtained that irregularity in composition which caused it to crys- 

 tallize out as rocks of such diverse composition as the mass 

 cooled down. 



The appearance in the field, however, in the opinion of the 

 present writer, supports Lacroix's contention that this narrow 

 band of "Roche Dioritique" was produced by the solution of 

 the limestone in the granite magma about the contact. Detailed 

 chemical studies and an accurate map of the area are, however, 

 required before this contention can be proved, and it seems pre- 

 mature to say, as Lacroix does, " L'evidence de la transformation 

 du granite par dissolution du calcaire est complete." 1 The 

 development of this dioritic contact facies seems, even in the 

 Pyrenees, to be unusual, and its volume as compared with 



'Le granite des Pyrenees et ses Phenomenes de Contact. Bull des Services de 

 la Carte Geol. de la France ; No. 64, p. 60. 



