Dr. F. H. Hatch—L, Silurian Felsites of 8.H. Ireland. 545 
II].—On toe Lower Sizturtan Fetsitrs or tHe Souts-Hast oF 
IRELAND.! 
By Freperick H. Hatcu, Pu.D., F.G.8. 
(Communicated by permission of the Director-General of the Geological Survey.) 
HE felsites of the south-east of Ireland are shown on the maps 
of the Geological Survey? to extend over considerable areas in 
counties Wicklow, Wexford and Waterford. Like the Welsh felsites 
they are contemporaneous with Lower Silurian (Ordovician) strata, 
and were probably erupted on an old sea-bottom. 'They are accom- 
panied by abundant deposits of tuffs and breccias, the component 
‘fragments of which consist mainly of felsite. 
A chemical examination of one of the felsites, made when I was 
preparing a petrographical description of the more important igneous 
rocks for a memoir on Sheets 138 and 1389 of the Irish Geological 
Survey, showed it to be asoda-felsite ;* and this discovery induced me 
to make a more extended examination of these felsites. A number of 
specimens from different localities were collected and sliced; and 
a chemical analysis of those that promised interesting results was 
kindly undertaken by Mr. J. Hort Player (to whom I here tender 
my best thanks). The object of this paper is to communicate these 
results and to discuss them in connection with the microscopic 
characters of the rocks. 
The main point brought out by chemical analysis is the almost 
entire absence of lime, and the presence of potash and soda in 
varying proportions : in other words, the lime-soda series of felspars 
is unrepresented in these rocks, which must therefore contain either 
a varying proportion of potash-felspar (orthoclase) and soda-felspar 
(albite) or one or more felspars of a potash-soda series (anorthoclase). 
According to the relative proportion of potash and soda the rocks 
are roughly separable into the three following groups: 
(1.) Those in which there is a large excess of potash over soda, 
the latter being present only in small quantity. These may be 
termed potash-felsites. 
(1I.) Those in which the soda, though present in considerable 
quantity, is yet subordinate to the potash. These may be termed 
potash-soda-felsites. 
(1II.) Those in which the soda is in excess. These may be termed 
soda-felsites.* 
1 Read at the Newcastle Meeting of the British Association, Sept. 13th, 1889. 
2 Sheets 180, 189, 148, 149, 158, 167, 168, 169, 178, and 179. 
3 Grou. Mac. 1889, p. 70. 
4 Should the soda be i a “but slight excess of the potash, the rock might be termed a 
soda-potash-felsite. The term keratophyre (originally suggested by Giimbel) has been 
applied by Lossen to a rather indefinite group aleve includes rocks similar in 
character to those in Group III. The term soda-felsite appears more applicable to 
these rocks. 
DECADE III.—yYOL. VI.—NO. XII. 30 
