[ 568 J 
XLVII. 
ON THE PROGRESSIVE DYNAMO-METAMORPHISM OF A 
PORPHYRITIC ANDESITE FROM COUNTY WICKLOW. 
By HENRY J. SEYMOUR, B.A., F.G.S. 
(PratEs XXVI. anp XXVII.) 
[COMMUNICATED BY PERMISSION OF THE DIRECTOR OF H. M. 
GEOLOGICAL SURVEY. | 
[Read Marcu 19; Received for Publication Marcn 21; Published June 28th, 1902.] 
THE object of the present communication is to describe an in- 
teresting case of the transformation of a porphyritic basic rock 
into a banded schist.1 It is also put forward as an exceptionally 
clear example of an already recognised possible mode of origin of 
banded gneisses, by the effects of pressure and recrystallization 
acting on suitable non-homogeneous rock masses or complexes. 
As has already been pointed out in another publication,’ 
numerous outcrops of basic rocks occur within a zone, about a mile 
wide, along the western flanks of the Leinster granite between 
Brittas and Baltinglass. These rocks were formerly mapped as 
ashes (Ds.) on the Survey maps (old editions), their apparently 
bedded character—really a shear structure produced by pressure— 
being regarded by the earlier observers as pointing to a sedimentary 
origin. {They are now mainly in the condition of hornblende and 
mica schists, and epidiorites, and were very probably pyroxenic 
varieties in their original condition, as is indicated by those 
examples which, owing to their having been outside the sphere of 
influence of the granite, have retained more or less their original 
structures. 
One such example occurs, amongst other places at Ballina- 
scorney Gap, west of Glenasmole, thin sections of the rock from this 
1T am indebted to my colleague Mr A. McHenry, for bringing under my notice 
the rock now described. 
* « Summary of Progress,’’ Geological Survey, 1899, pp. 71, 72, and 176, 
