ROCKS ALTERED BY VOLCANIC DIKES. 
515 
ample, near Plas-Kewydd, in Anglesea, has been described 
by Professor Henslow.^' The dike is 134 feet wide, and con¬ 
sists of a rock which is a compound of feldspar and augite 
(dolerite of some authors). Strata of shale and argillaceous 
limestone, through which it cuts j^erpendicularly, are altered 
to a distance of 30, or even, in some places, of 35 feet from 
the edge of the dike. The shale, as it approaches the trap, 
becomes gradually more compact, and is most indurated 
where nearest the junction. Here it loses part of its schis¬ 
tose structure, but the separation into parallel layers is still 
discernible. In several places the shale is converted into 
hard porcelanous jasper. In the most hardened part of the 
mass the fossil shells, principally Producti^ are nearly obliter¬ 
ated; yet even here their impressions may frequently be 
traced. The argillaceous limestone undergoes analogous 
mutations, losing its earthy texture as it approaches the 
dike, and becoming granular and crystalline. But the most 
extraordinary phenomenon is the appearance in the shale of 
numerous crystals of analcime and garnet, which are dis¬ 
tinctly confined to those portions of the rock affected by the 
dike.f Some garnets contain as much as 20 per cent, of 
lime, which they may have derived from the decomposition 
of the fossil shells or Prodiicti. The same mineral has been 
observed, under very analogous circumstances, in High Tees- 
dale, by Professor Sedgwick, where it also occurs in shale 
and limestone, altered by basalt. J 
Antrim: Dike cutting through Chcdk ,—In several parts of 
the county of Antrim, in the north of Ireland, chalk with 
flints is traversed by basaltic dikes. The chalk is there con¬ 
verted into granular marble near the basalt, the change 
sometimes extending eight or ten feet from the wall of the 
dike, being greatest near the point of contact, and thence 
gradually decreasing till it becomes evanescent. “ The ex¬ 
treme effect,” says Dr. Berger, presents a dark brown crys¬ 
talline limestone, the crystals running in flakes as large as 
those of coarse primitive {nietamorphic) limestone; the next 
state is saccharine, then fine grained and arenaceous; a com¬ 
pact variety, having a porcelanous aspect and a bluish-gray 
color, succeeds: this, towards the outer edge, becomes yel¬ 
lowish-white, and insensibly graduates into the unaltered 
chalk. The flints in the altered chalk usually assume a gray 
yellowish color.”§ All traces of org^ic remains are effaced 
in that part of the limestone which is most crystalline, 
* Cambridge Transactions, vol. i., p. 402, 
t Ibid., vol. i., p. 410. t Ibid., Vol. ii., p. 175. 
§ Dr. Berger, Geol. Trans., 1st. ser., vol. iii., p. 172. 
