PETROGRAPHICAL NOTES. 89 



e. — Tuffs , etc. 



In most localities where there is any great development of 



volcanic strata, the true igneous lavas are accom- 



Tuffs ' panied by great thicknesses of sedimentary tuffs, 



consisting of volcanic fragments redistributed by water. These 



tuffs often surpass in importance the lavas themselves. In a region 



like the one under examination, where the lavas themselves are 



considerably altered, it is to be expected that changes at least as 



great would take place amongst these much looser materials. The 



great thicknesses of chlorite schists which have 



Chlorite schists. . , . ., 



already been alluded to as accompanying these 

 lavas may be looked upon as the altered representatives of these 

 tuffs. The secondary mineral chlorite represents the altered ferro- 

 magnesian silicates, and owing to the great development of this 

 mineral, the rocks in this closely folded district have become com- 

 pletely schistose, and all trace of the original structure has dis- 

 appeared. These rocks usually contain calcile in such a large 

 proportion as to deserve the name of chlorite limestone or chlorite 

 cipollino. The calcite may be largely derived from the alteration of 

 the original minerals, but it results partly from simultaneous deposi- 

 tion, for the schists contain lenticular masses of limestone interleaved 

 between their folia?, and massive limestone beds are frequently 

 intercalated. (See above, page 69.) As already mentioned the 

 calcite itself may be volcanic in origin, the limestones being of the 

 nature of travertine. 



These chlorite schists often contain both pyrites and magnetite. 

 The pyrites crystals are sometimes of fairly large size and they 

 occur in the form of cubes with striated faces, as is very common 

 with that mineral. They are sometimes largely altered to hematite 

 Pyrites altered to without, however, the slightest change in their 

 hematite. crystalline shape. The smaller crystals are 



complete pseudomorphs, while the larger ones have an outer shell 

 of hematite that sends irregular branching extensions into the 

 inner mass of the crystal, much of which still consists of pyrites. 



( 89 ) 



