140 POSITION AND NATURE OF [Ch. VII. 



disposition of which, like that of trap veins in similar cases, 

 has accidentally coincided with the direction of the beds." 



Hornstone and felspar porphyries frequently occur in the 

 course of the Tilt on both sides of the valley. These some- 

 times appear to be veins, and at other times assume the form 

 of beds: but it is difficult to ascertain their true nature. 

 When the porphyry is parallel with the strata, resembling a 

 bed, it has oftentimes the appearance of alternating with the 

 schist, quartz-rock, and limestone. 



The excellent and circumstantial description of the junction 

 of granite and the limestone series in Glen Tilt has led us 

 to make longer extracts than was anticipated ; but the im- 

 portance of the subject required that the nature of this 

 connection should be fully and clearly detailed. Examples 

 of a similar description are of frequent occurrence in the 

 Alps and Pyrenees]; with this difference, that the fossiliferous 

 strata with which the limestone is connected are of a more 

 recent formation. In these countries, however, we cannot 

 obtain such distinct and minute displays of these rocks at 

 their actual contact ; but, on the other hand, the limestone of 

 these mountainous regions, which are crystalline, and imme- 

 diately associated with the granite, are often of less extent, 

 and, consequently, the secondary strata, into which they appear 

 to pass, are not far removed from the granite. These phe- 

 nomena furnish some of the most important evidences in 

 favour of the plutonic theory, and, therefore, it is necessary to 

 subjoin to these lengthened details at least one example from 

 the eastern side of the Pyrenees, as described by Dufrenoy. 



" Near Saint Paul de Fenouillet the limestone is associated 

 with a black marl enclosing fossils which belong to the 

 inferior beds of chalk, and are disseminated in the form of 

 black casts imbedded in a bluish-grey crystalline limestone : 

 these remains are not easily detected, unless a great number 

 of them have been examined ; for they appear to have been 

 so compressed, and adhere so strongly to the limestone, that 

 it is difficult to separate even characteristic fragments. From 



