274 Dr. Mac Culloch on the Geology of Glen Tilt. 



and to ascertain the real nature of this kind of alternation, since 

 It has been quoted at different times as proving a regular alternation 

 between granite and schist, the limestone, as it happens, having been 

 here overlooked. It is no further an alternation than because these 

 substances, which in numerous other instances are confounded and 

 irregularly mixed together, happen in this particular one and for a 

 very short space, to have assumed a disposition accidentally more re- 

 gular.* The question of alternation must rest on other facts than 

 these. A blank of about eighty yards follows this rock, and is suc- 

 ceeded by one of the most remarkable junctions of the different rocks 

 already described, which occurs in the course of the river. The 

 main body of this compound mass consists of white limestone or 

 marble, without any tendency to that regularly bedded form which 

 is the general characteristic of the limestone in Glen Tilt. It is ac- 

 companied by a small portion of schist as well as by a mass of granite, 

 with both of which it is variously intermixed. The marble itself 

 is in most places of a pure white, a fine grain, and dry aspect, and is 

 extremely hard. In some places it is of an ochry colour, and is in- 

 terspersed by thin veins of the same substance, so as to resem- 

 ble some of the palest and worst specimens of that marble known 

 by the name of Giallo Antico. The schist which accompanies it is 

 argillaceous and of a bluish colour, and the granite, both in the 

 larger masses and in the ramifications, bears so near a resemblance to 

 that formerly described that it is superfluous to describe it again. 

 The veins which traverse the marble, like those at the bridge above- 

 mentioned, are of various sizes and are placed in every possible di- 

 rection, but the ramifications are neither so numerous nor so minute 

 as in that instance, nor is there any appearance of the laminated 



* Vide Plate 14. 



