196 PROF. T. G. BONNEY ON CRYSTALLINE SCHISTS AHD THEIR 



examination is increased by the abundance of a brownish to black- 

 ish material, ferruginous or carbonaceous, which, so to say, stains 

 the rock ; but the matrix itself appears to consist mainly of small 

 flakes of white mica and granules, often elongated in form, of quartz 

 or of some silicate. There are no signs of strain-slip cleavage. The 

 present microfoliation seems due cither to original sedimentation or 

 to pressure acting on a material which has not possessed a very 

 definite structure. Oblong crystals of a silicate, often about -01" 

 by 'OOS", are not uncommon, usually lying with their longer 

 axes at a high angle with the structure of the matrix ; these are 

 posterior in date to the latter, because the dark lines indicative of it 

 can be traced through them ; sometimes they are rendered partially 

 or wholly opaque by reason of the dark matter. When clear they giye 

 low polarization-tints, and are often twinned on the Carlsbad type ; 

 one section gives extinctions of 14°-25 and 17°"25 respectively on 

 either side of the twin line, so they are probably monoclinic. I 

 have seen similar crystals in other Alpine phyllites, and occasionally 

 in much crushed schists, but am not able to identify the mineral 

 with any species known to me by name. After comparison of the 

 glide with specimens of typical phyllites from other parts of the 

 Alps, the Ardennes, &c., I think we must refer the rock to the same 

 group. 



This identification is confirmed by an examination of specimens 

 from the rocks pierced by the St. Gothard tunnel, which runs nearly 

 under the marble-quarr}'. The section disclosed in this reveals a 

 still more extraordinary state of things than has been just described. 

 It is well illustrated by the collection of specimens procured for the 

 British Museum from the late Dr. Stapf, which has been carefully 

 described by him *. In the tunnel the space between the two 

 gneisses mentioned above seems to measure about 270 yards. In 

 this interval we find that the following rocks are present : — 

 (1) phyllite, (2) marble and calc-mica schists, (3) phyllite, (4) calc- 

 mica schists, in which occurs a curious brecciated rock, (5) phyllite, 

 followed by soft rotten-looking calcareous rock (? representing 

 rauchwacke). From the distances at which the specimens are col- 

 lected, I conclude that the phyllites, &c., are not so thick as the 

 crystalline calcareous rocks. The specimens of the former come 

 from distances of 2562, 2637, and 2766-2790 metres from the north 

 entrance. They seem to owe their schistose condition to pressure : 

 the latter group are clearly ordinary calc-schists or calc-mica schists 

 more or less affected by subsequent pressure. 



Strange, therefore, as the association above described may be, 1 

 can come to iio other conclusion than that we have in this section 

 a newer argillaceous sediment and an older mass of crystalline 

 (calcareous) schists faulted together (in process, doubtless, of folding), 

 so that the apparent sequence is misleading, and that pressure, as is 

 commonly the case, has affected the softer argillaceous sediment 

 more than the more solid crystalline masses. I am, howevci', 



* I am indebted to Mr. Fletcher for great facilities afforded me in the study 

 of these, and to Mr. Miers for much kind help. 



I 



