156 TEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



of flow prove that they were derived from the shattering of already 

 consolidated rocks. In other places the ejected materials consist 

 almost wholly of black shale fragments, but with an intermixture 

 of felsite-lapilli. 



It is difficult to convey an adequate idea of the way in which 

 the agglomerates are traversed by dykes, veins, and bosses of 

 various felsites, and of how these break in endless confusion through 

 each other. Some of these intrusive rocks are compact and amor- 

 phous, others are vesicular, others close-grained aud columnar. 

 Again and again they present the most perfect flow-structure, aud 

 it is noticeable that the lines of flow follow the inequalities of the 

 walls of the fissure up which the rock has ascended, and not only so, 

 but even of the surfaces of detached blocks of shale or felsite which 

 have been caught up and enclosed in the still moving mass. 



A few of these intrusive rocks have been examined in thin slices 

 by Dr. Hatch. Most of them appear to be soda-felsites, but they 

 include also rather decomposed rocks, some of which are probably 

 diorites and quartz-diorites. Occasionally, thoroughly basic dykes 

 (dolerite) may be observed. 



In the midst of this tumultuous assemblage of volcanic masses, 

 representing the roots of a group of ancient vents, there occur 

 occasional interspaces occupied by ordinary stratified rocks. In the 

 eastern j)art of the section these consist mainly of black shale, some- 

 times with calcareous bands, from which a series of Bala fossils 

 has been obtained *. A very cursory examination suffices to show 

 that these intercalations do not mark pauses in the volcanic erup- 

 tions. They are, in fact, portions of the marine accumulations under 

 the sea-floor through which the vents were blown ; they have been 

 tossed about, crushed, and invaded by dykes and veins of felsite. 



But certain other intercalated strips of stratified rocks present a 

 special interest, for they bring before us examples of volcanic ashes 

 that gathered on the sea-floor, but which were disrupted by later 

 explosions. Thus, at the Knockmahon headland, well-bedded 

 felspathic grits and ashy shales occur, thrown in among the general 

 mass of eruptive material. As I have already remarked, it is 

 difficult or impossible to fix the horizons of the stratified patches 

 that are involved among the igneous ejections of this coast-section, 

 save where they contain recognizable fossils, but the intercalation 



* But see the Geol. Survey Memoir on Sheets 167, 168, 178, and 179, Ire- 

 land (1865), p. 28, for a description of the association of Bala and Llandeilo 

 fossils on that coast-line. 



