1 l6 PROCEEDINGS OF THE GEOLOGICAL SOCIETT. 



overlying mass. The " greenstones " may therefore be regarded as 

 lateral emanations from funnels of more basic lava towards the 

 close of the volcanic period. Possibly some at least of the highly 

 slaggy and vesicular bands to which I have referred may represent 

 portions of this material, which actually flowed out as streams of 

 lava at the surface. 



Eiit there is likewise evidence of extensive intrusion of the more 

 siliceous rocks. On the Geological-Survey map, besides the numerous 

 " greenstones/' various sheets of " felspathic porphyry " are repre- 

 sented as running with the general strike of the region, but here 

 and there breaking across it. One of the most remarkable of these 

 acid sills is that which iu the noble precipice of Cader Idris has a 

 thickness of about 1500 feet and a length of three or four miles. 

 It is shown on the map to be transgressive across other rocks and, 

 as seen on the ground, it maintains the uniformity of texture which 

 is characteristic rather of sheets that have solidified underneath 

 than of those which have congealed with comparative rapidity at 

 the surface. On a fresh fracture the rock presents a pale bluish-grey 

 tint, becoming yellowish or brownish as the result of weathering. 

 Its texture is finely granular, with occasional disseminated felspars. 

 Under the microscope a section of it was found by Dr. Hatch to 

 exhibit the characteristic structure of a microgranite, a confused 

 holocrystalline aggregate of quartz and felspar, with a few porphy- 

 ritic felspars. Messrs. Cole and Jennings have proposed to revive 

 for this rock Daubuisson's name " Eurite " *. 



A similar rock occurs at a lower horizon among the Lingula Flags 

 at Gelli-llwyd-fawr, two miles south-west of Dolgelly t, and much 

 microgranite has been injected along the slopes above Tyddyn-mawr. 



With these and other intrusions the granite should perhaps be 

 considered which rises as an oblong boss-like mass a little to the 

 north of Eestiniog. 



I am not aware that the chronological relation of these acid 

 sheets and bosses to the more basic intrusions has yet been definitely 

 determined. That some of them may have solidified in vents and 

 may have been directly connected with the protrusion of the later 

 or more highly siliceous lavas is not at all improbable. 



The remarkable scarcity of dykes in the volcanic districts of 

 Wales has been noticed by more than one observer. Among the 

 intrusive *' greenstones " of Merionethshire some occasionjilly 



* Mr. Harker speaks of the rock as a granophyre. 

 t Cole and Jennings, op. cit. p. 435. 



