THEORY. 441 



east, and underlies the Taconic slates, etc. May not the magnesian slate have been 

 lifted up with these slates upon its back ? If so, then the Taconic slates must be the 

 newest. But one more argument may be adduced. The Taconic slates appear to dip 

 beneath the magnesian slates upon their west side. Therefore if this is the true position, 

 they must appear upon the east side of the magnesian slate, if they belong beneath it. 

 But unequivocally the magnesian slate is underlaid by the Stockbridge limestone, and 

 that by the granular quartz. Hence we have these two horns of the dilemma either 

 the clay slates belong beneath the granular quartz, or above the magnesian slate. The 

 former horn certainly cannot be taken, consequently there is but one horn left; that is, the 

 Taconic slates are newer than the magnesian slates. 



It is important to determine the upper limit of the Taconic system, and ascertain what 

 rocks overlie it. Fortunately in Vermont and vicinity there are beautiful examples of 

 the junction of the Taconic and Silurian systems. Go to Whitehall, NVY., in sight of 

 Vermont, where the opponents of the Taconic system have derived so much comfort. 

 The Potsdam sandstone there rests directly upon the Laurentian rocks, unconformably, 

 for it is by the unconformability alone that we can determine the limits of the two 

 systems. (See description of Fig. 168.) The anti-Taconic theorist says, what can you 

 mean by asserting the presence of a system of rocks 25,000 feet thick between the 

 Potsdam sandstone and the Laurentian system, when there is not a single foot of it at 

 Whitehall between these two systems ! Why, we mean, says the Taconic advocate, 

 precisely what you would say of the relative position of the Champlain clays to the 

 Potsdam sandstone and gneiss ; and on the very same ground, too, in Whitehall. You 

 admit that the Champlain clay rests upon both the sandstone and the gneiss. Why does 

 not the sandstone intervene between the clay and the gneiss ? Simply because the sand- 

 stone cannot cover the gneiss everywhere, owing to the laws of the formation of rocks. 

 The rocks do not surround the globe in concentric coats of sediment. Each system has 

 its own locality. Consequently there must be some line where rocks of two different ages 

 unite. What now is there to hinder a subsequent deposit to be placed, so that it shall 

 cover the edges of the two different formations ? Indeed it is an admitted fact that the 

 clay covers both the gneiss and the sandstone. And all that is maintained in respect to 

 the relations of the Potsdam sandstone to the Taconic and Laurentian rocks is, that it 

 covers the edges of both, where they unite. The Taconic rocks were deposited in the 

 great valley between the Laurentian rocks of New England and New York. The western 

 edge must be somewhere. The top of the highest member, that is, the western edge of the 

 system, is about three miles east of Whitehall. After the deposition of the black slate, 

 the Potsdam sandstone is to be laid down. Where will the currents deposit the sand ? 

 Obviously very near the black slate, the last band formed previously ; just as the Trenton 

 limestone is always formed near the Chazy limestone. So the Potsdam sandstone was 

 laid down upon a floor partly of Taconic, and partly of Laurentian rocks, and near the 

 highest member of the Taconic system. 



Those who wish, will find the junction of the Potsdam sandstone and black slate east of 



Whitehall. Go two or three miles east of the village of Whitehall, upon the east side of 



the mountain, and the immediate line of contact is exposed in a deep ravine. Thus the 



sandstone is merely an outlier. There is another similar exposure of the calciferous sand- 



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