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196 PROF. BONNEY ON THE RELATION OF BREccTAs [May 1902, 
the mass a black speckled look. In these interstratified breccias 
the rock-fragments generally vary from about 3 inches in diameter 
downward, and are mixed up without regard to size, but in 
places a large boulder occurs: for instance, one or two (in volume 
3 or 4 cubic feet) may be found in a mass where the bulk of the 
fragments do not exceed 5 inches in diameter, and are mostly much 
smaller. On the left bank of the Raverettaz stream we speedily come 
(near Aigremont) to the great mass of breccia.’ This is exposed, 
forming a cliff, higher up the mountain, and the road, on emerging 
from the ravine, crosses its outcrop... Beyond this are found two 
or three comparatively thin beds of breccia, one of which contains 
a block of granite some 5 yards long. Afterwards the road passes 
over grassy slopes till it arrives at Orment Dessus. 
This mass of breccia, according to Prof. Renevier and Dr. Schardt,? 
can be traced for about 5 kilometres to the south-east and 2 kilo- 
metres to the north-west, but it may extend much farther in this 
direction ; the latter observer gives it a breadth of 6 to 7 Wlometres 
from north-east to south-west, the blocks diminishing in size towards 
the east, north, and west, but increasing towards the south, where 
they are lost beneath overthrust Jurassic strata. The fragments 
in the Flysch, he remarks, vary with the locality. 
The materials of the breccias must now be briefly noticed. In the 
great mass, mentioned above, the matrix is a dark gritty mudstone, 
the volume of which is less than that of the fragments. Of these the 
majority are less than 5 inches in diameter, but the rest may be of any 
size up to several cubic feet, the largest being some variety of granite 
orgneiss. Crystalline rocks predominate in the fragments, many 
of them showing pressure-foliation ; but among the sedimentaries I 
noticed a dark limestone and dark shaly or slaty rocks, some of the 
latter containing fragmeutal mica (? from the Carboniferous system). 
Vein-quartz is common. ‘The following kinds of rock are enumer- 
ated by Messrs. Favre & Schardt* as occurring in the Aigremont 
breccia:—(a) At least eight varieties of granite, greyish or green- 
ish, fine and coarse; (6) at least three varieties of gneiss ; (¢) mica- 
schist, with white to yellowish mica; (d) green schist, with more or 
less chlorite; (e) green talcose clay-schist ; (f) quartzite; (q) green 
petrosilex or felsite; (h) dark slaty rocks, generally in small bits ; 
(‘) grey limestones, sometimes in large blocks. My own obser- 
vations, though less minute, are in accordance with this. Some of 
the green schist reminded me of members of the ‘ Griiner Schiefer ’ 
sroup elsewhere in the Alps, but occasional fragments were un-_ 
usually bright in colour. A few, more compact, recalled a ‘ petro- 
silex ’ that I had found near the Pissevache waterfall (Vernayaz). 
The microscopic structure of some of these rocks may now be 
briefly described, for it is needless to dwell on the more minute 
1 The road crosses a ‘ berg-fall,’ which occurred in the 16th century. 
2 So far as I could estimate from this, the thickness must exceed 100 feet. 
3 I am much indebted to these gentlemen for kindly sending me particulars, 
‘in reply to my letter of enquiry. 
4 «Beitrage z. geol. Karte d. Schweiz’ vol. xxii (1887) p. 204. 
