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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Although this is the general character of the rock, it frequently 

 assumes other appearances. It is either compact and silicious, or 

 it is argillaceous and soft ; it has sometimes the appearance of 

 hornstone or of quartz-rock, and it is at times altered into a mass 

 resembling porphyry. This porphyritic appearance is well seen 

 in the ravine behind the -workshops of La Cava, where a portion 

 of it has assumed a quartzose or hornstone character — if indeed 

 this is not the result of veins of serpentine penetrating the meta- 

 morphic rock. 



Notwithstanding various opinions which have been advanced, 

 I have no doubt that the spheroidal forms assumed by this rock 

 are the result of the conditions under which the cooling and 

 induration of the mass took place, after it had been reduced 

 by plutonic agency to a fused and liquid state. It bears an ana- 

 logy to the formation of basaltic columns, indicating certain 

 isolated points in which the cooling process, accompanied by cer- 

 tain principles of aggregation, commenced, extending its influence 

 in all directions until interrupted by similar efforts proceeding 

 from other points. 



Macigno. 



Gabbro rosso. 



Serpentine. 



I have said above that this red gabbro is an altered rock ; let us 

 consider from what parent rock it has been derived. In the ravine 

 to the south of the workshops and the mine of La Cava, is a good 

 section of the gabbro rosso and true superincumbent beds, showing 

 a gradual passage from the spheroidal masses into thin stratified 

 highly contorted beds, dipping off at an angle of 50° or 60° to the 

 south as represented in the above diagram. These beds are of the 

 same dark liver-red colour, and break into similar small brittle 

 masses, which at first cannot be distinguished from broken hand spe- 

 cimens. As we recede from the central mass of gabbro, the difference 

 becomes more perceptible. Some of the beds are harder than others ; 

 and, on obtaining a real fracture, exhibit the appearance of a quart- 

 zose grit. Others are friable and gritty, and easily crumble to 

 pieces, with the appearance of a half-baked brick. By degrees as- 

 cending the series, a few patches of grey colour occur in them, and 

 this gradually increases. They are twisted and contorted in a 

 most extraordinary manner, being turned completely over in some 

 places like the mica schists and indurated marls of the primary 

 and palaeozoic formations, indicating the violence of the actions by 

 which they were upheaved. As we get further from this sup- 



