ON THE FORMATION OF CRYSTALLINE ROCKS. 265 



a great extent chlorite, in innumerable microscopic crystals,* has 

 been formed between their lamellas ; feldspar has also sometimes been 

 introduced ; furthermore, a multitude of quartz veins, some parallel 

 to the lamellae and others oblique, have been isolated within them, and 

 these veins themselves often contain the minerals which have just 

 been cited ; lastly, sandstones have been changed into quartzites.f 

 Now we can hardly admit that stratified and fossiliferous formations 

 could have originally possessed those mineralogical characters, and 

 hence it is conceded by all that they owe their present nature to a 

 transformation undergone since their deposition. 



But when the same phenomenon presents itself under a more ad- 

 vanced phase, it needs a more attentive examination to authenticate 

 it, nor is it always possible to attain a certainty, because the primi- 

 tive type has been more or less completely effaced by chemical action 

 posterior to the formation of the sedimentary rock. Thus, in the 

 immense masses of crystalline rock of the Alps, we find, as in the 

 Ardennes, a chlorite schist with veins of quartz and often of chlorite, 

 but it is in general more perfectly crystallized. (Zillerthal, in Tyrol, 

 Salsbourg.) It is associated with a series of other crystalline rocks, of 

 various natures, which alternate irregularly among themselves, espe- 

 cially talcose schist, green schist, ."t: amphibolic schists, and even some 

 schistose diorites,§ talcose gneiss, (described by Saussure under the 

 name of veined granite,) quartzite,|| calcareous and often micaceous 

 schists, more rarely dolomites and gypsum, also studded with a variety 

 of minerals. (Environs of Airolo.)*|[ Yet, notwithstanding the pe- 

 culiarly crystalline character of the rock, the greater part of the ge- 

 ologists who have described the Alps have considered them as of 

 sedimentary origin. 



The conclusion that certain crystalline and very extended forma- 

 tions, such as those of the Alps, are metamorphic, rests upon many 

 evidences which are, however, very nearly of the same order with 

 those which prove the metamorphism that has taken place in the 

 neighborhood of eruptive rocks. I mention the following : 



1. The analogy of composition Avhich unites certain groups of 

 crystalline with sedimentary rocks is at this day still striking, not- 

 withstanding the modifications which the first appear to have under- 

 gone. We find, in effect, as in the sedimentary rocks, beds of limestone, 



•■' M. Sauvage discovered by analysis the existence of chlorite, even in those varieties of 

 phyllades where the naked eye cannot distinguish it. It is under these same conditions 

 that sericite is found in the schists of Taunus. 



t Explication de la Carte Geologique de France, vol. i, p. 77. Durocher ; memoir cited above, 

 p. 603. 



J The rocks named green tchists by Mr. Studre, and which are certainly metamorphic, 

 have been recently carefully examined by Mr. Hath. They are of a very variable compo- 

 gition, and often contain aligoclase and albite. — [Zeitschrift der Deutsch. Geol. Gesclhchaft, vol. 

 ix, p. 211.) 



§ We have before mentioned that the schistose diorites of the Uial are generally consid- 

 ered us metamorphic. 



II Transactions of the Geological Society of London, 1st series, vol. iv, p. 26i ; 2d series, vol. 

 i, p. 53 ; Cosmos, vol. i, p. 305. 



^Serpentine itself, in certain of its formations in the Alps, the Ural, the Alleghanies, 

 and elsewhere, appears to result from the metamorphism of different amphibolic and 

 other rocks, as numerous observations prove. 



