252 
THE SILURIAN VOLCANOES 
BOOK IV 
The rocks iu each of these three areas are similar. One of tlieir dis- 
tinguishing features is the intercalation among them of a fossiliferous lime- 
stone and calcareous fossiliferous tuff's, which contain well-preserved species 
of organisms characteristic of the Bala division of the Lower Silurian rocks.^ 
There cannot he any question that these organisms were living at the time 
the strata in which their remains occur are found. The most delicate 
parts of the sculpture on lUcenus llowmanni and OrtMs elegcmtula are well 
preserved, ^or have the limestones been pushed into their present places 
by volcanic agency, or by laults in the terrestrial crust. They are not oiily 
regularly intercalated among the volcanic rocks, but the limestone in some 
places abounds iu volcanic dust, while above it come calcareous tuff's, also con- 
taining the same fossils. It is thus clearly established that the volcanic 
series now to he descrilied has its geological age definitely fixed as that of 
the Bala period. 
Tlie lavas ot the Lough Mask region consist of felsites and andesites 
with rocks of probably more basic composition. The felsites are generally 
quartziferous porphyries, which occupy a considerable space in each of the 
thi’ee districts. To what extent they are intrusive rather than interstrati- 
fied remains for in^■estigation. Some of them have undoubtedly invaded 
other members of tlie volcanic series. But, on the otlier hand, fragments 
ol similar quartz-porphyries and felsites abound in the intercalated bands of 
volcanic breccia. 
The andesites and more liasic lavas are finely-crystalline or compact, dull- 
green to chocolate-purple rocks, often resembling the “ porphyrites ” of the 
Old Bed Sandstone. Some of them are strongly vesicular, the cavities being 
filled with calcite on tresh fracture, though empty on weathered surfaces. 
The sack-like or pillow structure, already referred to as characteristic of many 
Lower Silurian lavas, appears conspicuously among some of these rocks. At 
Bohaun, nine miles south from Westport, where a prolongation of the volcanic 
series rises to the surface from under the overlying coarse conglomerates, I 
observed that, owing to the compression which the rocks have there 
undergone, the pillow-shaped blocks have been squeezed together into rudely 
polygonal forms, while their vesicles have been greatly drawn out in the 
direction of tension. Where the rocks have been still more sheared, the 
distinct pillow-shaped blocks with their vesicular structure disappear, while 
the more fine-grained crusts that surround them have been broken up and 
appear as fragments iinulved in a matrix of green schist. 
Intercalated with the lavas are numerous bands of volcanic breccia and 
fine tuff. The stones in these breccias consist chiefly of A arious felsites with 
andesites and more basic lavas. But pieces of jasper, chert, shale and grit 
are not infrequent. In some places abundant blocks of black shale are to be 
noticed, probably derived from the Llandeilo group which exists below, and 
which lias here and there Iieen ridged up to the surface in the midst of the 
^ See tlie li.st of fossils as determined by Mr. Baily in Explanatory Memoir to accompany Sheets 
73, 74, 83 and 84 of the Geological Survey of Ireland, p. 68 (1876). 
