378 



development of a silicate-hornfels from a limestone the carbonic acid 

 becomes replaced by silicic acid and probably certain bases are introduced, 

 though detailed chemical evidence is required before the latter statement 

 can be confidently made. 



So far we have been considering cases in which the rocks subjected 

 to metamorphism are fine-grained and more or less argillaceous or 

 calcareous in character. We have now to consider the alteration of 

 arenaceous rocks. The most important communication on this subject 

 is by M. C. BARROIS. (I) The granite (granite with two micas, or granulite 

 of French authors) of Guemene' produces important alteration on the yrfa 

 a scolithes. The unaltered rock is a light-coloured sandstone composed 

 essentially of quartz and white mica. The quartz occurs in rounded 

 or irregular grains which are nearly uniform in size ('010 to '012 mm). 

 Scales of sericitic mica are distributed irregularly through the mass. 

 Grains of zircon about '08 mm in diameter are present in all the micro- 

 scopic slides and there is also a certain amount of argillaceous or ferru- 

 ginous matter without any definite characters. In the neighbourhood of 

 the granite the sandstone becomes harder and darker in colour. In the 

 first stage of metamorphism a dark mica makes its appearance and the 

 quartz-grains lose all traces of a clastic origin. The rock becomes a 

 micaceous quartzite. The dark mica is similar in character to that so 

 frequently found in rocks which have suffered contact-metamorphism. 

 It is usually without definite crystalline form. The individual scales vary 

 in size in different specimens, the largest being found nearest the granite. 

 They occur as a rule between the quartz-grains as if the recrystallization 

 of the silica had in most cases preceded the development of the dark mica. 

 Sometimes, however, they occur as inclusions in the quartz. The liquid 

 cavities are arranged in lines in the quartz-grains and these lines may 

 sometimes be followed from one grain to another without a break in their 

 continuity. The dark mica of the micaceous quartzites appears to have 

 been developed at the expense of the sericite and of the argillaceous and 

 ferruginous materials of the original sandstone. Nearer the granite 

 sillimanite makes its appearance in the quartzite (quartzites micaces 

 sillimanitises) and is sometimes sufficiently abundant to give a silky aspect 

 to the rock. It occurs in the form of acicular crystals which are often 

 grouped in sheaves and bundles. The thickness of the needles is always 

 small and never exceeds '01 to '02 mm. Crystalline faces are not definitely 

 recognizable. The needles are striated parallel to their lengths and are 

 traversed at irregular intervals by cross joints. These joints are some- 

 times seen to stand in direct relation to lines of liquid inclusions in the 

 neighbouring quartz-grains. The sillimanite is colourless. It gives vivid 

 tints under crossed nicols and extinguishes in all cases parallel to the long 

 axis. Sometimes the sillimanite has been replaced by fine micaceous fibres. 

 Magnetite occurs in grains and more rarely in definite crystals. It results 

 from a change in the ferruginous matter present in the original rock. 



(1) Sur les gres metamorphique du massif granitique du Guemene. Ann. d. 1. Soc. 

 Geol. d. Nora, T. XI., p. 103. 



