240 
THE SILURIAN VOLCANOES 
BOOK IV 
probably of Lower Silurian age, which I have referred to (p. 201) as wedo-ed 
in between the schists and the Old Led Sandstone along the southern margin 
of the Higlilands of Scotland, reappears in Ireland. It occupies an area in 
the County Tyrone, about 24 miles long and about 9 miles broad at the 
broadest part, but disappearing towards the north-east and south- west. ^ 
>ying between the Paheozoic formations on the south and the schists on 
the north, it occupies a similar position to the Scottish belt, but presents 
a much broader area, and thus affords greater facilities for examining 
the rocks. It presents the same indefinite or fiiulted boundaries as in 
Scotland, so that its relations to the rocks along its flanks have not been 
satisfactorily determined. That the rocks of this area are older than the 
Silurian strata to the south of them seems to be established by the occur- 
rence of fragments of them in these strata, and that they are younger than 
the schists may be inferred from their non-foliated character. But they 
have undoubtedly undergone con.siderable crushing by powerful terrestrial 
movements which have placed them in their present position. 
The special feature of interest in this Irisli area is the remarkable 
development of volcanic materials which is there to be seen, spreading over 
a for wider area than in Scotland. The rocks include lavas associated with 
tuffs and agglomerates, likewise a varied series of intrusive masses. 
The lavas are chiefly dull greenish, fine-grained rocks, having the general 
character of diabases and “ porphyrites.” They are sometimes quite slaggy, 
and where the amygdaloidal kernels remain, these are usually of calcite' 
Under the microscope, the diabases show in some parts that their lath- 
shaped felspars, and the augite which these penetrate, are tolerably fresh, 
while in other parts fibrous chlorite, granular epidote and veins of calcite 
bear witness to the metamorphisin which they have undergone. 
One of the most conspicuous features in some of these lavas is the 
occurrence of the same sack-like or pillow-shaped structure which has been 
ali'eady referred to as so marked among the Arenig lavas of Scotland. 
Though the vesicles of these rocks are often quite uncrushed, showino- that 
there has been no general subsequent deformation of the whole mass°there 
occur local tracts where evidence of considerable movement may be noticed. 
Thus close to a mass of gneiss, and elsewhere along their margin, the lavas 
are apt to be much jointed and broken with numerous lines of shear, aloiuT 
which the crushed material assumes more or less of a schistose structure! 
Yet m the solid cores between these bands of crushing the original forms of 
the vesicles are retained. ” 
Ihese greenish lavas are occasionally interleaved with grey flinty mud- 
stones, cherts and red jaspers, which are more particularly developed imme- 
diately above. In lithological character, and in their relation to the 
in Purvey, and was described by him 
A tv I “y colleagues, Mr. B. N. Peach and Mr. 
. M Heniy, 1890 and again m 1894. My first conclusion was that the volcanic rocks should 
771 ‘'^^00 n ® ^™s lying to the north of them (Pres. Address Gcol. Soc. 
^ i ^ tte rooks of the border of the Scottish 
Highlands, I formed the opinion stated in the text. 
