192 The American Geologist. September, looi. 
and always more or less schistose. It separates more easily 
into layers than the foregoing. Plagioclase feldspars occur in 
both, in the latter in large white crystals. "All forms of pass- 
age between these two types are found." Thus, the enlarge- 
ment of the feldspar crystals of the gneissic type transforms 
the rock into a true augen-gneiss the large feldspars of which 
with square sections sometimes measure five millimeters in 
length by two or three in width, and are alligned parallel. The 
color of the rock then becomes lighter, and the fragments that 
it contains more separated. Then the orientation of the feld- 
spars disappears, the schistosity diminishes more and more, 
and the feldspathic element is dispoesd in all directions with 
respect to itself, and there is formed a rock still granitic, but 
of a type almost prophyroidal. At the same time the basic 
element marks out in the rock real lines or trains analogous to 
thtjsp of particles carried in suspension in a viscous liquid." 
(2) These two types of granitic rock are not capriciously 
and irregularly distributed in the midst of the protogine. The 
true granitic type fomis the southern slope of the massif, the 
col du Geant, the Grandes Jorasses, monuts Roux, the Dolent, 
the peaks of Planereuse, the Portelet, the summits of the 
Chatelet and of Breya, where, further, the grain of the rock 
becomes remarkably fine. But the gneissic varieties on the 
contrary are found in the neighborhood of contact of the pro- 
togine with the schists, and also in the central area of the massif 
where they appear to be arranged in lines along more or less 
definite axes, viz : between the Grandes Aiguilles and the sum- 
mit of the Grandes Jorasses (p. 21). While the authors de- 
clare it is impossible to define the limits of the different parts of 
the gneiss, owing to the transitions to granite, as mentioned, 
they state that in general along certain axes the gneissic belts 
are manifest and separated one from the other by zones in 
which is evolved the granitic type. Further, intercalated in 
the midst of the gneiss are bands of real mica schist. In total, 
therefore, the mountain is composed of two great zones, more 
granitic than gneissic, enclosing between them a region where 
the protogine is more gneissic and schistose, a region which cor- 
responds to a great central depression. 
(3) The mica schists are accompanied by amphiboly tes. 
(4) While there are distinct apophyses of the protogine 
