268 EXPERIMENTS ON METAMORPHISM AND 



hand, some silurian formations are hardly modified at all, even in their 

 inferior beds, as we see in Russia, Sweden, and in the United States.* 

 The remarkably crystalline state of many pala3ozoic formations 

 should not, then, be exclusively attributed, as has been supposed, to 

 a certain peculiar position in which the earth was at the time of their 

 deposition, but in reality to peculiar agencies which have affected 

 certain regions in preference to others. 



CHAPTER III. 



METAMORPHISM OF STRUCTURE. 



Many masses of rock can be more or less perfectly divided into 

 parallel lamellge.t These lamella3 are not caused by a cleavage of crys- 

 tallization, nor are they owing to stratification. The plane of the 

 lamellge is frequently oblique to that of the beds. There are coun- 

 tries, however, where the transversal disposition is exceptional, and 

 where the lamellas are generally parallel to the stratification. | 



This lamellar structure is particularly developed in argilaceous 

 schists or phyllades, but it is not exclusively peculiar to them; it be- 

 longs to rocks of difi"erent natures, such as quartzites, sandstones, 

 limestones, especially when they are impure. Different circumstances 

 show that lamellar rocks have undergone mechanical action, particu- 

 larly energetic pressure, which has produced upon them indelible 

 effects. The greater part of the fossils which they contain have been 

 compressed and drawn out in a very characteristic manner. It is to 

 the slips which have resulted from this pressure that the lamellar 

 structure appears to owe its origin, as is confirmed by the experi- 

 ment which we shall presently cite. 



Certain peculiarities of structure, less marked than the cleavage, 

 result likewise, without doubt, from mechanical action. Such are the 

 secondary joints, known to those who v^^ork slate ;§ the fibrous structure 

 which results as if from a folding of the lamellasjil the structure called 

 pseudo-regular, which occurs frequently in quartzites and coal. These 

 different methods of division are therefore to be also mentioned as a 

 metamorphism of the kind we are describing. The abnormal schistose 



~' According to the very recent observations of Sir R. Murchison, metamorphism has 

 played a more important part than was suspected in the constitution of the silurian for- 

 mation of Scotland. 



f Often the joint is not more distinct in tlie rock before it is brought out b)' a shock than 

 cleavage in crystals ; it is, as it were, latent, as we see in the slate quarries. 



% This habitual parallelism has been noticed in theHartz, in Saxony, in Britanny, in Scot- 

 land, in Devonshire, and in the system of the Rhine, by Hausniann, Naumann, Durocher, 

 Macculoch, De la B^che, Baur, and De Dechen. 



§ The chief of these secondary joints is called longrain by the quarrymen of the Ar- 

 dennes. 



II The bacillary structure of some limestones of the Alps, such as those of Kiam, in the 

 Tyrol, is an example of it. — (Favre; Giologiedu Tyrol Allemandc; Bibliotheque de Oenhe, 1849.) 



De la B^che has given examples of these divisions in bis Geological Eev&rt of Cornwall, 

 p. 271. 



