ANNIVERSARY ADDRESS OF THF PRESIDENT. I I 3 



arc interbeddcd lavas or intrusive sheets. Dr. Hatch finds that 

 their microscojnc characters show a close resemblance to the soda- 

 felsites described by him from the Bala series of the south-cast of 

 Ireland. 



The remarkably cellular rock which forms the peak of Cader 

 Idris is coloured on the Survey map as an intrusive sill of *' green- 

 stone," which in the Memoir is said to alter the contiguous slates 

 and to appear to cut across them diagonally. I am disposed, how- 

 ev(T, to think that these appearances of intrusion are possibly de- 

 ceptive. On the southern declivity of the mountain this rock pre- 

 sents one of the most curious structures to be seen in the whole 

 district. Its surface displays a mass of spheroidal blocks aggregated 

 together, each having a tendency to divide internally into prisms 

 which diverge from the outside towards the centre. Some portions 

 are extremely slaggy, and round these more solid portions finely 

 crystalline parts are drawn, suggestive rather of free subaerial motion 

 than of the conditions under which a subterranean sill must be formed. 

 The idea occurred to me on the ground that while the band of rock 

 marked as "greenstone " on the map is probably, in the main, an 

 interstratified lava, there may nevertheless be basic intrusions along 

 its course, as in the lower part of the mountain. 



The minute structure of this amygdaloid, as revealed by the 

 microscope, shows it to be an epidiorite wherein the hornblende, 

 pararaorphic after augite, has been again partially altered along the 

 margins into chlorite. 



The highest lavas of Cader Idris, forming the ridge to the south of 

 Llyn Cau, are separated from the amygdaloid just described by a 

 thick zone of black slate wnth thin ashy intercalations, beyond which 

 comes the coarse volcanic agglomerate already referred to as con- 

 taining blocks of felsite a yard or more in diameter. These lavas 

 are true felsites, sometimes beautifully spherulitic and exhibiting 

 abundant flow-structure, like some of the felsites of the next or 

 Bala volcanic period *. The petrography of these rocks still remains 

 to be worked out. 



Hardly any information has yet been obtained as to the situation 



* Cole and Jennings. Quart. Journ. Geol. See. vol. xlv. (1889) p. 430. From 

 the examination of slices prepared from a few of the felsites of the Dolgeliy 

 district, Dr. Hatch lias observed a ' striking difference between their characters 

 and those of the Cambrian felsites of Caernarvonshire. Tiie porphjritic con- 

 stituent is now no longer quartz but felspar (plagioclase), and the rocks belong 

 not to the rhyolitic but rather to the less acid trachytes, perhaps even to the 

 andesites.' 



