Vol. 67.] SKOMER VOLCANIC SERIES. 213 



all probability to the same epoch of volcanic activity. From 

 Mevagissey, in the south of Cornwall, Dr. Teall had recognized 

 diabases, pillow-lavas, keratophryres, and soda-felsites belonging to 

 a series which also was Lower Silurian. The continuation of 

 these rocks to the south-west had been investigated by Mr. J. B. 

 Hill and the speaker on the northern boundary of the Lizard 

 district ; and there again was an assemblage of diabases, pillow- 

 lavas, and 'soda-felsites,' the last-named being rather coarse-grained, 

 but having a composition very similar to some of the Author's 

 rocks. Hence, it seemed established that one characteristic of the 

 eruptions of this period was the development from one magma of 

 basic types of rock (diabases, etc.) along with acid and intermediate 

 types that were especially rich in soda-felspar. 



The rocks called marloesites and skomerites by the Author 

 were of great importance, as examples of igneous rocks of rather 

 basic composition in which the dominant felspars contained a high 

 proportion of albite. Long- continued investigation of the older basic 

 rocks of Britain had convinced the speaker that in a vast number 

 of cases the only felspar which, they contained was albite. In part, 

 this mineral was no doubt primary, but in large measure it was of 

 secondary origin. Albitization, or the replacement of other felspars, 

 especially lime-felspars, by albite was a process that had gone on in 

 Britain on a very large scale. In Cornwall and Devon, for example, 

 by far the greater number of the ' diabases ' had been completely 

 albitized. It was by no means easy to lay down the principles on 

 which it could be determined whether the soda-felspar in any one 

 of these Palaeozoic igneous rocks was primary or secondary. But the 

 speaker had had an opportunity of examining the Author's slides 

 of marloesite and skomerite, and he agreed that the alkali-felspars 

 of these rocks were for the most part original ; and, although albiti- 

 zation had taken place to some extent, especially in the phenocrysts, 

 it had not essentially modified the characters of the rocks. 



The recognition of mugearites in Skomer Island was a welcome 

 addition to our knowledge of British petrography. First established 

 by Mr. Harker, only seven years ago, to include a limited number 

 of Tertiary rocks in the island of Skye, this group had proved to 

 have a wide distribution in the Carboniferous volcanic areas of 

 Scotland. Mr. Barrow had found them in East Lothian, Mr. Bailey 

 and the speaker in Midlothian, and Mr. Tyrrell in the Glasgow 

 district ; now, for the first time, they had been discovered in Wales. 



Mr. Evan Small was glad to find that the observations made 

 some years ago by Mr. F. T. Howard and himself had been largely 

 confirmed. The Author had, however, very greatly extended these 

 observations, and he congratulated him. Referring to Mr. Green's 

 remarks, as to a nodular lava on Ramsey Island being mistaken 

 for a sedimentary conglomerate, he noted that a similar error had 

 been made by earlier observers on Skomer : for, on the old Geological 

 Survey map, the whole of the spherulitic rhyolite of Tom's House 

 and the Basin in the south of the island was shown as sedimentary. 

 The true character of this rock was, however, recognized so long 



