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part. Quartz-veins are abundant where the disturbance has been 

 very great. Under the microscope the rock is seen to consist princi- 

 pally of quartz, often in more or less rounded grains, sericite in 

 minute irregular scales, hematite in grains and hexagonal tables, 

 carbonaceous matter and needles of a yellow or reddish yellow 

 mineral (? rutile). The accessory minerals are pyrite, tourmaline and 

 chlorite. As the granite masses are approached the rock changes in 

 character. First of all dark spots make their appearance in the 

 normal clay slate, then the matrix of the rock becomes more 

 distinctly crystalline and the schistosity becomes less perfect. Still 

 nearer the granite the spots become less and less marked ; while in 

 the innermost zone the rock is usually massive and spots are no 

 longer recognizable. The term hornfels is applied to the rock of 

 the innermost zone. Professor ROSENBUSCH divides the aureole of 

 metamorphic rock into three zones which are not, however, separated 

 from each other by any hard and fast lines. These three zones are 

 designated as follows : 



(1) Zone of spotted clay-slate (knotenthonschiefer). 



(2) Zone of spotted mica-slate (knotenglimmerschiefer). 



(3) Zone of hornfels/ 1 ) 



The spotted clay-slate differs from the ordinary clay-slate in 

 showing, on a fractured surface, a large number of small dark spots. 

 These spots vary in size from that of a pin's head to that of a 

 pea. Under the microscope the constituents of the spotted slates 

 are seen to be essentially the same as those of the unaltered rock. 

 Magnetite is, however, somewhat more abundant. The spots are 

 merely portions of the slate which are somewhat richer in dark 

 pigment (iron oxides and graphite) than the surrounding mass. 



The spotted mica-slate differs from the above in being more 

 distinctly crystalline and somewhat less fissile. The constituents 

 show no trace of clastic origin under the microscope. The hematite 

 of the original rock has entirely disappeared and its place has been 

 taken by magnetite which often occurs in well-crystallized octahedra. 

 In addition to the constituents already mentioned there occur brown 

 mica and staurolite. The brown mica appears to take the place 

 of the chlorite in the unaltered rock. The staurolite occurs in 

 very small crystals (largest '02mm). It may be recognized by its 

 optical characters, including pleochroism, and by the fact that the 

 prismatic angle is about 130. The spots in the mica-slate are finer 

 in grain than the matrix in which they lie. Their outlines are 

 often ill-defined and finally, as the hornfels-zone is approached, they 

 disappear altogether. 



The rock of the innermost zone is usually massive in appearance 

 and with scarcely a trace of schistosity in any direction. It is usually 



(1) We use the term hornf els instead of its etymological equivalent, hornstone, because in 

 many cases the rocks termed hornfels are distinctly crystalline and not at all like what is 

 commonly termed hornstone in this country. 



