Professor Favre on the Geology of the Germcm Tyrol. 77 



of prismatic distinct concretions, and fitting to each other, 

 as may be seen in small billets of wood split in the 

 direction of the fibres; only these small concretions leave no 

 empty spaces, and sometimes become so slender that the 

 rock "resembles hard asbestus. This state is the result of the 

 maximum of development in the circumstances which pro- 

 duced the slaty structure ;— it is that sti'ucture carried to the 

 extreme. Certain crystalline slates present some resem- 

 blance in their structure to this limestone. Only, these rocks 

 being formed of many mineral substances, the structure is 

 not so regular. We notice in it nodules, and imperfect cry- 

 stals of quartz or felspar, which are enclosed in cavities 

 more or less deep, lined by one of the substances of the slate 

 in leaflets (mica, talc, or steatite). This substance appears 

 to have been subjected to friction, for it is marked with 

 small striiB on the surface. The greater part of the rocks 

 known under the name of satin-slates present this same 

 character. 



In a question so complicated as that of the origin of the 

 slaty structure, we ought not to have recourse to a single 

 action in order to explain it. Accordingly, it is admitted 

 that, in certain cases, this structure has been produced by 

 the fusion of rocks, which have flowed downwards ;* and that, 

 in other circumstances, it represents the remains of stratifi- 

 cation. Lastly, we may perceive further, that this pheno- 

 menon owes its origin to abrasions and frictions,t which have 

 taken place before the complete solidification of the rock. This 

 latter mode of formation appears to me evidently demon- 

 strated by the resemblance of the crystalline slates to the 

 rocks of Klam. The diff'erent forms of these limestones, 

 taken in connection with the observation made at the en- 

 trance of the Zemm-Thal, appear to us to indicate that the 

 slaty structure, in the case of which we speak, is the result 

 of frictions and etirements. To this it must be added, that the 



* Naumann. (Neues .lahrbuch fur Minera., 1847, p. 297. Archives do la 

 IHbl. Univ., t. vii., p. 322; 1848.) 



t Studer. (Memoir formerly referred to). Pournet (Annales de la SocieKJ 

 d' Agriculture de Lyon, t. iv. ; 1841 ct 1846.) 



