484 DR. C. CALLAWAY ON DERIVED FRAGMENTS IN THE 



gneissoid rocks. Prof. Bonney considers that one of the last-named 

 varieties " almost certainly exhibits a gneissoid structure due to 

 crushing and cementation anterior to the making of the pebble.*' 



199. Some of the above kinds, with mica-schist and a banded 

 gneiss. In this slide, as in one or two of the preceding, is a rock 

 consisting of a little quartz, some decomposed felspar, and a large 

 proportion of a chloritic mineral. It reminds me of some altered 

 diorites I have seen ; but Prof. Bonney declines to decide whether 

 it is igneous or metamorphic. 



200. Pebbly grit, containing fragments of the altered grit, quartz- 

 schist, and mica-schist, as before. 



These specimens point to the pre- existence of metamorphic rocks 

 of three types : — 



(1) Coarse-grained gneissic and granitoid rocks like those in the 

 Wrekin chain, at the Ercal and Primrose Hill, and in the Malvern 

 HiUs. 



(2) Quartzites, quartz-schists, and mica-schists of characters 

 intermediate between (1) and (3) — that is to say, of finer grain and, 

 in the case of the schists, of clearer foliation than (1) — but exhi- 

 biting no trace of an original clastic structure. Comparing them 

 with rocks with which I am personally acquainted, I find they most 

 closely resemble some of the schists of the Kilmacrenan series * in 

 Donegal, and certain types which I have recently examined in 

 Connemara. 



(3) Hypometamorphic grit. The clastic structure in this rock is 

 quite distinct ; but the matrix has been more or less altered into 

 crystalline minerals. Omitting cases of contact metamorphism, I 

 do not know of any rocks of this kind except amongst the newer 

 Archsean series of Anglesey, the Lough-Eoyle group, and some of 

 the Archsean rocks near Wexford. I have also detected at "Westport, 

 CO. Mayo, altered grits which suggest this rock. 



The occurrence of rounded fragments of this grit in the Charlton- 

 Hill conglomerate cannot be a case of contemporaneous denudation, 

 since, on every probable theory of metamorphism known to us, the 

 alteration could not have been produced at the surface. That the 

 change took place subsequent to the inclusion of the pebbles in the 

 conglomerate is improbable, for the matrix of the conglomerate 

 itself does not display mineralization. The presence of this grit in 

 an Archsean conglomerate is a datum of great interest. 



The bearing of the above facts upon the Archsean controversy is 

 obvious. In the Shropshire area, in times anterior to the Cambrian, 

 there existed, in addition to the volcanic series, three groups of 

 rocks displaying a close lithological resemblance respectively to (1) 

 the Hebridean and Malvernian gneisses ; (2) the intermediate 

 schists of Ireland, Sutherland, and Anglesey ; and (3) the hypo- 

 metamorphic grits of Anglesey and Ireland. All of these Salopian 

 rocks had undergone more or less metamorphism prior to the 

 outburst of the later Archaean volcanoes. The Charlton-Hill con- 



* Quart. Journ, Geol, See. vol. xh. p. 230. 



