660 STRATIGRAPHICAL GEOLOGY. 



age, unless there is reason of importance to the contrary. For example, 

 we trace one calcareous rock continuously for over 150 miles. There are 

 slight modifications in its appearance from place to place, and still more 

 so when we compare the specimens taken from the most extreme locali- 

 ties. I refer to the case of the Calciferous mica schist. In Canada a 

 considerable portion of the series is purely a limestone, with occasional 

 spangles of mica scattered through it. At the other extreme, in Franklin 

 county, Mass., the limestone is reduced to occasional strata, a few inches 

 thick, of micaceous character, perhaps containing 50 per cent, of calcium 

 carbonate, while 90 per cent, of the formation consists of a mica schist. 

 This modification of character is greater than is usually insisted upon for 

 New Hampshire. The style of similarity, made use of for identification, 

 is better shown in the porphyritic gneisses. There are over thirty areas 

 of porphyritic gneiss, in which the feldspar crystals are very conspicuous 

 for their size, the rock being the Augen gneiss of Europe. I assume that 

 all the areas of this rock are identical in age, and, in speculating upon the 

 relative positions of the intervening groups, rely upon the correctness of 

 this starting-point. A thorough study of the specimens from the several 

 porphyritic areas shows there are unimportant but uniform differences 

 between them, so that boulders which have been transported from ten to 

 fifty miles can be referred directly to their particular source, one or 

 another of these areas. The fact of minor differences would seem to 

 confirm our assumption of their identity in age, just as the paleontolo- 

 gist finds, from the presence of the same fossils, proof of contempora- 

 neity in rocks with dissimilar mineral character. 



Of still greater consequence is the method in which flexures are dis- 

 posed. The following are some of their peculiarities in this field: i. Sym- 

 metry of flexure is common, but by no means universal. Our sections 

 are made much more even in the drawings than is natural. The elevat- 

 ing forces have crushed the strata together roughly, and we may not find 

 the same thickness of strata upon both sides of an axis. 2. Hence the 

 tenderest formations, like slates, are the ones most disarranged, the hard 

 bunches of granite or gneiss being incompressible, and forcing the others 

 out of place. 3. The vertical position does not necessarily indicate 

 greater antiquity than an adjacent inclination of a much smaller angle. 

 This appears also from the fact that the less inclined formation may 



