375 



of a dark blue colour. Mica-flakes are generally recognizable. Some 

 varieties are hornstone-like ; many are, however, distinctly crystalline in 

 appearance. The term hornfels is applied not only to the compact but 

 also to the more or less crystalline varieties. It is used by many writers 

 as a general name for the rock of the innermost zone in regions of 

 contact-metamorphism, and is thus applied even to schistose and banded 

 rocks. The schistose varieties are, however, generally designated as 

 schistose hornfels. The dominant rock of the innermost zone in the 

 Vosges contains andalusite, and is therefore termed andalusite-hornfels. 

 It consists of quartz, two micas, andalusite, magnetite and hematite. 

 Brown mica (phlogooite) is more abundant than in the preceding zone. 

 Magnetite occurs in grains and octahedra and is more abundant than 

 hematite. Andalusite occurs in grains and crystals with more or less 

 rounded angles. The crystals are usually short microscopic prisms showing 

 the forms (110) and (001). Cross sections extinguish parallel with the 

 diagonals; longitudinal sections give straight extinction. The mineral 

 is often colourless in thin section. The coloured sections show the usual 

 pleochroism. In some varieties of the rock the crystals attain a con- 

 siderable size and thus give rise to the porphyritic texture. In the banded 

 (gneiss-like) hornfels layers rich in andalusite alternate with others in 

 which this mineral is comparatively rare. The constituents of the ordinary 

 massive (not banded) hornfels are arranged without any regularity in 

 the rock-mass. 



Prof. ROSENBUSCH describes also some local and exceptional varieties 

 of hornfels, under the names cordierite-hornfels, garnet-hornfels and tour- 

 maline-hornfels. The cordierite-hornfels contains irregular grains of 

 cordierite (iolite), in which the usual inclusions of sillimanite occur. 

 Hornblende and augite, in irregular grains, but possessing characteristic 

 cleavages, were also observed in one locality. The smaller inclusions in 

 the cordierite are surrounded by yellow pleochroic borders, which dis- 

 appear on heating a section on platinum-foil in the flame of a Bunsen's 

 burner, and which are accordingly supposed to be due to an organic 

 colouring matter. 



The garnet-hornfels contains garnet, pyroxene, magnetite, hematite, 

 sphene and a colourless mica. It possesses a specific gravity of 3'024, 

 and is evidently the result of the metamorphosis of a bed exceptionally 

 rich in calcareous matter. The tourmaline-hornfels is schistose in struc- 

 ture. It is composed of tourmaline, staurolite, white mica and quartz. 



The memoir of Prof. ROSENBUSCH contains an elaborate series of 

 analyses of the different varieties of altered and unaltered rocks. These 

 analyses show beyond all doubt that, except in the case of the tourmaline- 

 hornfels, there has been no important alteration in the chemical compo- 

 sition of the rock. The metamorphosis has merely been accompanied 

 by a more or less complete molecular rearrangement of the original 

 constituents. The junctions with the igneous rock are sharp, and there 

 is not the slightest evidence of a passage from the altered sediment to 

 the eruptive granite. 



