Dr. Mac C u l l o c h on the Geology of Glen Tilt. 27 3 



marble, and exceedingly indurated, may be seen occupying a space 

 of about twenty feet. If a careless view of these beds be taken, they 

 will be found to offer what appears for the space of about ten feet, 

 to be an alternation of granite in a very regular disposition with 

 limestone. It has been mistaken for a real alternation. If this rock 

 be more carefully examined it will be evident that it consists of a 

 mere fragment, and that it is abruptly cut off where it meets the 

 hill on the right, by abutting against the mass of granite. As the 

 lower end dips under the water, a clear view of that part was not to 

 be obtained when I examined it, but it appeared there also to be cut 

 off by a granite vein. Those parts therefore of the granite which 

 seem to alternate with the limestone can only be considered as 

 portions of veins, the disposition of which, like that of trap veins 

 in similar cases, has accidentally coincided with the direction of 

 the limestone beds. If granite does really occur in beds, I know 

 no reason to prevent it from alternating with limestone, but the 

 appearance here is much too limited, and too doubtful in its origin 

 to be admitted as an instance of such alternation. The gra- 

 nite which accompanies this junction may be seen in the bed 

 of the river for seventy or eighty yards, after which it disappears. 

 Three or four hundred yards of a blank alluvial space occur immedi- 

 ately after this, followed by a rocky space of about 200 yards. 

 This rocky portion exhibits the following alternation, granite, schist, 

 granite, limestone, succeeded by a general confusion of all these 

 substances, and the granite which is in general limited to the right 

 bank, now crosses to the left of the river. When" I say that there 

 is such ^n alternation, I do not mean that the several substances are 

 bedded in this order, the case to which the term alternation is more, 

 properly applied. It is particularly necessary to attend to this remark. 



Vol. III. 2 m 



