Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/281

 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 of 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 of 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 granite 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 immediately 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 an 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,