I 



ON THE VARIOLITIC ROCKS OP MONT GENKVRE. 303 



seen to consist of actiuolite, uralite, and common hornblende. The 

 hornblende forms long frayed-out masses, with the fibres bent and 

 broken, and often surrounded by a zone of radiating needles of 

 actinolite. As a rule, the green amphibolic areas are quite irre- 

 gular in shape, and often consist of a felt, or of radiating masses of 

 acti n olite-needle s . 



The third constituent is titanic iron, and this also is decomposing 

 and passing into leucoxene. 



Neither olivine nor pseudomorphs after it appear to be present, 

 and this forms another of the points of resemblance between this 

 rock and the saussuritic Cornish gabbros, which rarely contain 

 olivine *. 



Along its western margins on this spur of Le Chenaillet the rock 

 is of finer grain, and it has not only been altered by the develop- 

 ment of microlites and decomposition-products, but by the mechanical 

 rearrangement of the constituents. In the first stage the rock is 

 schistified into an eye-gabbro, strikingly like that of Karaclews. 

 The primary felspar is turbid and opaque, and both it and the 

 pyroxene occur in eyes or lenticles drawn out along the plane of 

 schistosit5^ Between these are layers of green hornblende and a 

 mosaic of clearer secondary plagioclase. Aetinolite-needles are 

 greatly developed, and occur in lines sweeping round the felspar and 

 pyroxene. 



Still closer to the margin of the mass occurs an interesting fissile 

 rock. The felspar, which largely predominates, is far less turbid ; 

 the pyroxene has all been altered to actinolite which, with the 

 sphene associated with it, occurs in straight lines through the 

 rock. 



Ascending the ridge, we cross many diabase -dykes in the gabbro, 

 and finally come to a sudden junction with the rocks of finer grain 

 that form the main mass of the ridge. From Lory's description Ave 

 had expected a gradual passage from one to the other ; on the con- 

 trary, the junction is very sharp, and probably faulted. On the 

 west side of a line that runs straight across the hill, the gabbro is 

 at its coarsest ; on the other side are the normal diabases with 

 variolitic selvages. There is no sign whatever of a passage : the 

 general appearance of the junction, which, however, is not well 

 shown, and the extent to which the rocks have there been slicken- 

 sided, both suggest a fault. 



There are several other outcrops of the same coarse-grained gabbro 

 in the area. Thus a little to the north, along the line of junction of 

 the igneous rocks with the calc-schists, it forms the face of the slope 

 of the platform that runs along beneath the ridge. It is the same 

 coarse-grained mixture of " saussurite " and '* snuiragdite," traversed 

 by diabase-dykes and serpentinous patches, which will be subse- 

 quently described. In the upper part of the west Chenaillet valley 

 there is another exposure which extends from the lowest lake to the 

 north bank of the dry basin of a former tarn, where it abuts against 



^ J. J. ir. Teall, * Britisli Pelrograpby,' London : 188S, p. 177. 



