160 ME. E. E. COWPEE EEED ON THE GEOLOGY OP [May 1895? 



wholly composed of fragments of the greenish and grey lava-flows 

 which crop out immediately to the east as far as ' North Pole ' 

 Cottage. A few pieces of black slate are seen in this coarse agglo- 

 merate of volcanic fragments, which is entirely different in com- 

 position from that exposed at Goodwick. We cannot doubt that 

 we have here an old volcanic pipe, filled with the rocks shattered 

 by the last explosions, similar to the pipes that are so common 

 amongst the Upper Palaeozoic rocks of Scotland. 



The types of lava particularly noticeable at this horizon on 

 Strumble Head are a pale greyish rock and a yellowish-green rock 

 dotted with milk-white spots, and fragments of these compose the 

 breccia. 



The general appearance of most of the lavas in the field is flinty,, 

 with a more or less perfect conchoidal fracture. It is impossible to 

 mention any constant features by which they can be distinguished 

 from the finer compacted tuffs in the field, for they weather in a 

 similar fashion, and even with the aid of the microscope the dis- 

 crimination is frequently a matter of great difficulty and uncertainty. 

 On a fresh fracture the lavas show considerable differences in colour, 

 being in some cases pale grey, in others dark bluish-grey, in others 

 again olive-green, and in yet others pale yellowish-green. They 

 may all be included under the term ' felsite.' Their specific gravity 

 ranges from 2*60 to 2*76. 



There are three special types of structure remarkably well deve- 

 loped in some felsites of this area, all of which are visible to the 

 naked eye in the field. Firstly, the banded structure is exhibited 

 with great distinctness in many of the lava-beds near Goodwick : 

 fine light and dark bands rapidly alternate one with the other, and 

 sometimes are thrown into folds and convolutions. Secondly, there 

 is the nodular structure. This has been found only at Penrhyn, 

 near Goodwick, and the pale grey or whitish ovate nodules, | inch 

 or so in length, give a peculiar blotched or spotty appearance to the 

 rock. They are frequently more or less fused together in strings, 

 and either have their centre filled with a clear quartz-grain or are 

 hollow. They weather out white in a dark-grey matrix, but the 

 nodules compose the chief bulk of the rock. Weathered fragments 

 and beach-pebbles of this peculiar lava bear a really striking resem- 

 blance to some species of the Carboniferous coral Lithostrotion. 

 Thirdly, there is the perlitic structure. It is rare to find this 

 macroscopically distinguishable, but on the weathered surface of the 

 lavas of Carn Gelli, south-west of Goodwick, it is beautifully showm 

 Under the microscope slices of rocks from many other localities in 

 this area show perlitic structure, as will be described farther on 

 (pp. 172-73). 



There are some rocks amongst the volcanic series which show a 

 curious mottled, blotchy, or streaky appearance to the naked eye.- 

 In many cases, as in the rock from Carn Pica, between Fishguard 

 and Dinas, it is practically certain that they are re-cemented lava- 

 flows, in which the fragments of the brecciated crust or congealed 



