ANNIVERSARY ADDRESS OF TUE PRESIDENT, 79 



into green chloritic schists. Some portions of them arc not unlike 

 the slaty tuffs of Llyn Padarn in Caernarvonshire. 



Accompanying the fragmental volcanic rocks some ordinary sedi- 

 mentary intercalations are to be found — red shales and pebbly 

 quartzites that seem to have escaped much crushing. 



Lastly, along the volcanic zone rise numerous craggy hills which 

 mark the positions of bands and bosses, probably intrusive like the 

 sills and bosses already noticed ; they consist of gabbros which have 

 been altered into epidiorite. 



We thus learn that the local base of the Dalradian schists in the 

 north of Ireland (probably not so low as the lower part of the series 

 in central Scotland) consists of a volcanic group wherein the lavas, 

 chiefly basic, show as distinct an amygdaloidal structure as can be 

 found in any younger I'ocks, while the fragmental materials include 

 coarse agglomerates and fine tuffs abounding in fragments of acid 

 as well as basic lavas. It is worthy of notice that in this early 

 volcanic group the two great classes of acid and basic rocks occurred 

 in the ejections from the same centre of eruption, and that these 

 ancient representatives of the two types resemble in essential 

 features those of later periods. The singular freshness of so many 

 of these rocks, and their escape in large measure from the shearing 

 which produced the schists, suggest at first that they cannot possibly 

 be part of the schistose series. But they are banded with true 

 schist, and have thus partially suffered deformation, while the 

 microscope reveals the large amount of internal change which they 

 have undergone. That they have not been affected to a greater 

 degree is possibly due to the protection afforded to them by the long 

 Archaean ridge from the influence of the gigantic pressure which 

 came from the south-east during the production of the foliation of 

 the schists. Another instance of a similar kind lies on the north- 

 west side of Ben Yuroch in Perthshire, where Mr. G. Barrow, of the 

 Geological Survey, has found that under the lee of the granite of 

 that mountain the limestone, which elsewhere has acquired the 

 structure of a marble, still retains its original character, while its 

 accompanying shales have not passed into schists. As is well known 

 also, the cleavage which is so intense on the east side of the long 

 quartz-porphyry ridge that crosses Llyn Padarn in Caernarvonshire 

 almost disappears on its western side. 



In the course of my investigation of the history of the earlier 

 volcanic periods in British geology it has been necessary for me to 



