330 THE GEOLOGY OF THE ALPS. [XIV. 



greater inclination is obtained if we choose, instead of these, 

 the summits of the Aiguilles which bear the same names, and, 

 although now isolated, represent portions of the former mass 

 of Mont Blanc. 



The crystalline rocks of this region present two types : first, 

 the protogines which form the centre ; and, second, the crys- 

 talline schists which occupy the flanks and form the Aiguilles 

 Rouges. These schists are also found at a great elevation on 

 the mountain; at the Grands Mulcts (4,666 metres) the rocks 

 are talcose and quartzose schists with graphite, hornblende, 

 epidote, talc, and asbestus, and similar rocks and minerals are 

 found from thence to the summit. The protogines themselves, 

 according to the evidence of nearly all who have studied them, 

 are stratified rocks, gneissic in structure, and pass in places 

 into more schistose varieties, though Favre regards the distinc- 

 tion between these and the crystalline schists proper as one 

 clearly marked. The outlines presented by the weathering of 

 the protogine are very unlike the rounded forms assumed by 

 true granite rocks. According to Delesse, the rock to which 

 Jurine gave the name of protogine is a talco-micaceous granite 

 or gneiss, made up of quartz, generally more or less grayish or 

 smoky in tint, with orthoclase, grayish or reddish in color, and 

 a white or greenish oligoclase with characteristic striae, often 

 penetrated with greenish talc. The mica (biotite), which some 

 previous observers had mistaken for chlorite, is dark green in 

 color, becoming of a reddish bronze by exposure. It is binaxial, 

 nearly anhydrous, and contains a large portion of ferric oxide. 

 The composition of the protogine rock, as a whole, differs from 

 that of ordinary granite, according to Delesse, only in the 

 presence of one or two hundredths of iron-oxide and magnesia. 

 The name of arkesine was given by Jurine to a variety of 

 protogine containing chlorite with hornblende, and sometimes 

 sphene. Among the other crystalline rocks of the Alps are 

 various talcose and chloritic schists, with steatites, chromifer- 

 ous serpentines, diallage rocks, diorites, and euphotides, asso- 

 ciated with beds of petrosilex or eurite, frequently porphyritic. 

 Highly micaceous schists, often quartzose, and holding garnet, 



