Seymour—WMetamorphism of a Porphyritic Andesite. 569 
locality showing that it was originally an augite-diorite, or dolerite 
without olivine.’ 
The amount of alteration produced on these rocks depends on 
their proximity to the granite. For example, the outcrops over a 
mile distant seem to be scarcely at all affected, while those some- 
what nearer have usually been more or less re-crystallized into 
amphibole rocks. Those, however, near the granite have been, in 
common with the associated sediments, not only highly thermo- 
metamorphosed, but also considerably sheared. It may be here 
_ mentioned that the comparatively narrow extent of the zone of 
alteration on the western side of the granite of the Leinster chain, 
as compared with that on the eastern side, is due probably to the 
much steeper descent of the granite beneath the slates on the for- 
mer side, as was first pointed out by Maxwell Close in 1877.” 
These basic rocks all probably belong to the same period, and occur 
apparently as intrusive sills, bosses, and dykes, in sediments 
believed to be of Lower Silurian (Ordovician) age. Some, how- 
ever, may possibly be lava flows. The porphyritic type occurs 
locally usually as dykes in the more compact varieties (andesites). 
A similar association occurs at Lambay and Portrane, and the 
general identity and mode of occurrence of the basic rocks of these 
localities, and of those on the west of the Leinster granite, suggest 
that all belong to the same geological period, namely, Baia, 
according to the recent work of Messrs. Gardiner and Reynolds.’ 
Though some interesting changes have been brought about in 
these older rocks by the granite, the present Paper is concerned 
only with one instance; having reference to some small outcrops in 
the neighbourhood of Donard (Co. Wicklow), noted at a and B on the 
accompanying sketch map of the district (p.570). Donard isa small 
village, some six miles south-east of Dunlavin ; and in the case of 
the outcrops just mentioned the results of both thermo- and dynamo- 
metamorphic action are well illustrated in the changes brought 
about in a porphyritic andesite developed here in some abundance. 
One outcrop of this rock occurs at Deerpark-hill (8), a little overa 
mile south of Donard, the other at Ballymooney-hill (a), about an 
1 « Summary of Progress,’’ 1899, p. 177. 
2 Journ. Roy. Geol. Soc. Ireland. vol. v., p. 57. 
3 Quart. Journ. Geol. Soc., vol. liv., p. 147, 1898. 
