510 THE POPULAR SCIENCE MONTHLY. 



ing to the most ancient systems, do not participate in these crys- 

 talline characters, but in their argillaceous rocks are similar to 

 those which are found in recent formations. This is the case in 

 Sweden, Russia, the United States, and Canada. But it has been 

 observed in those places that the strata have not been strongly 

 dislocated as in the regions we have just been speaking of, but 

 have retained their original horizontality. To this circumstance 

 they doubtless owe their preservation. The mineralogical con- 

 trast between formations of the same age corresponds, therefore, 

 with an essential difference in their bearing. 



There are countries where formations of less antiquity have 

 also suffered profound transformations. The Alps afford funda- 

 mental data on this subject. In the face of the rocks of different 

 ages — Carboniferous, Triassic, and Tertiary — which enter into their 

 composition, one is surprised at the special physiognomy which 

 each one of them presents as compared with what we observe in 

 beds of the same age in other regions, where they have remained 

 horizontal. A general influence has, therefore, acted upon a part 

 of the vast region of the Alps. It has affected rocks of every 

 epoch, even those of the Lower Tertiary — that is, a series of beds 

 many thousand metres thick — and that, although eruptive rocks 

 are very rare in it. 



With the mineralogical changes which we have just noticed is 

 associated a modification of texture that depends on the same 

 cause. It is well known in the slates as the schistose or lami- 

 nated structure. The fissile rocks which it characterize,^^ have the 

 property of detaching themselves in thin plates — that is, of cleav- 

 ing in certain directions. Observations made in various countries 

 have demonstrated the important fact that the planes of cleavage 

 are quite distinct from the planes of stratification. Instead of 

 being parallel to the layers, they are frequently oblique, and — 

 what is still more conclusive — while the planes of stratification 

 have been bent and exhibit a variety of inclinations, the planes of 

 cleavage pursue a regular direction, regardless of the most pro- 

 nounced inflections, and remain constantly parallel to one another. 

 This independence shows, besides, that the planes of cleavage 

 were produced, not only after the beds in which they are mani- 

 fest were deposited, but also after they had lost their primary 

 horizontality. The schistose disposition, very frequent in the 

 most ancient f ossilif erous rocks, sometimes persists in more recent 

 formations, when they have been subjected to energetic disloca- 

 tions. In many localities of the Alps slates are quarried into the 

 Tertiary formation. 



An important characteristic of the schistose rocks is the con- 

 siderable deformations which the fossils in them have received, 

 as is seen in the trilobites of the Angers slates. ISTot less fre- 



