Prof. T. G. Bonney—Quartz-Schists from the Alps. 209 
condition prior to Mesozoic times, and the uprearing of the present 
Alpine chain only rendered them more fissile and produced occasional 
flexures. 
But in none of these quartz-schists can original fragments be 
distinguished with certainty. Here and there a clastic structuic 
may be suspected. Here and there an inequality in size may sug- 
gest that the larger quartzes either are not authigenous or, if so in 
a certain sense, only represent the remains of grains which have 
escaped better than the rest in a general catastrophe. There is nothing 
to intimate that the mica in its present form is derivative. Like the 
quartz, it seems either to have formed or at least to have acquired 
its present outline im situ.' The few grains of felspar in one specimen 
most nearly resemble fragments. I cannot detect any structural 
difference, allowing for the altered proportion of the constituents, 
between these quartz-schists and some of the mica-schists, such as 
are found in the same group, and the latter correspond more nearly 
(though by no means identical) with certain laminated sediments 
altered by contact metamorphism” than with any other rocks known 
to me. 
But these quartz-schists are markedly different from ordinary 
quartzites.2 In these either the original sand-grains, though sur- 
rounded by an aureole of secondary quartz, can be readily recognized, 
or the comparatively regular granular structure suggests a clastic 
origin. So too does the mica, when it occurs. No marked recon- 
stitution of constituents has taken place. A felspar fragment still 
remains as a fragment; rotten perhaps, but not, as it were, inoscu- 
lating with a mosaic of quartz and white mica. These quarizites, 
also even after being subjected to pressure, do not put on, so far as I 
know, the aspect of quartz-schists‘—in the former the fragmental 
character of the rock, the cause of its modified structure, are at once 
apparent. Not so in the quartz-schists; that they were once sand- 
stone, though doubtless true, is only a matter of conjecture; how 
far they have been affected by pressure is a matter of uncertainty.’ 
1 The structure of these schists sometimes recalls that of certain of the rocks on 
the Hastern Gneiss near Kinlochewe, but the structure is coarser, the fragmental 
character (due in the latter to crushing) is less distinct, as if mineral reconstitution 
had been more complete in these Alpine rocks. 
* See Q.J.G.S. vol. xliv. p. 11. 
3 [ have not a few slides in my collection, representing, among others, the ‘ Dur- 
ness,’ Wrekin, Hartshill, Lickey, Stiperstones, Cherbourg, and Ardennes quartzsites. 
4 T am indebted to the kindness of Messrs. Peach and Goodchild, and the favour of 
Si A. Geikie, for the opportunity of examining a fine specimen of the ‘ piped’ 
quartzite (Basal Cambrian) from Glendhu, Sutherlandshire, which has become, 
through great pressure, slabby and schistose. It exhibits a structure with which I 
was already familiar from other rocks affected by the great thrust faults of the N.W. 
Highlands, and this is so different from that of the quartz-schists described in this 
paper that I could distinguish the one from the other at the first glance through the 
microscope. 
5 J except, of course, the cases where the rock exhibits flexures. Here it may be 
added that since the first notes of this paper were written I have had the opportunity 
of seeing the quartz-rocks near Clifden (Connemara)—a prolongation westward of 
the Twelve Pins. These in the field reminded me frequently of the Alpine quartz- 
schists, and always appeared to differ from the Durness quartzite; they seem to be 
DECADE III.—YVOL. X.—-NO. V. 14 
