ANMVERSARr ADDRESS OF THK PRESIDENT. Ill 



of iron, wliich was subsequently broken up so as to leave a residue 

 of minute cubes of magnetite*. 



Above tlie ironstone some more blue and black shale and grit pass 

 under a coarse volcanic conglomerate like that below, lying at 

 the base of 1 he high preciince of Cader Idris. Hence this inter- 

 calated group of sedimentary strata marks a pause in the discharge 

 of ashes and lavas, during which the peculiar conditions of sedi- 

 mentation indicated by the ironstone spread over at least the southern 

 part of the volcanic area. Some few miles to the east, where the 

 ironstone has been excavated near Cross Foxes, the band is again 

 found lying among tuffs and grits full of volcanic lapilli. 



between a lower and an upper band of tuff in the Arenig volcanic 

 group the maps and Memoir of the Geological Survey distinguish a 

 central zone of "felspathic porphyry," which attains a maximum thick- 

 ness of 1500 feet. From Sir Andrew Ramsay's descriptions, it is clear 

 that he recognized in this zone both intrusive and extrusive sheets, 

 and that the latter, where thickest, were not to be regarded as one 

 mighty lava-flow, but rather as the result of successive outpourings, 

 with occasional intervals marked by the intercalation of bands of slate 

 or of tuff. To a certain extent the intruded sheets are separated on 

 the map from the contemporaneous lavas ; but this has been done 

 only in a broad and sketchy way. One of the most important, and 

 at the same time most difficult, tasks yet to be accomplished in this 

 district is the separation of the rocks which were probably poured 

 out at the surface from those that were injected underneath it. My 

 own traverses of the ground have convinced me that good evidence 

 of superficial outflow may be found in tracts which have been mapped 

 as entirely intrusive ; while, on the other hand, some of the so-called 

 " lavas " may more probably be of the nature of sills. 



The petrography of the rocks, moreover, is as yet almost untouched. 

 Among the so-called " felspathic porphyries " of the Survey maps a 

 considerable variet)' of texture, structure, and composition will 

 doubtless be detected. In the " Descriptive Catalogue of Eoek- 

 Specimens in the Museum of Practical Geology " (3rd edition, 1862) 

 the rocks that form the " lava-streams of Llandeilo age " in Merio- 

 nethshire are named " felstone," " felspar-porphyry," " felstone- 

 porphyry," " felspathic porphyry," and " calcareous amygdaloid." 



The most interesting feature which my own slight personal ac- 

 quaintance with the ground has brought before me is tlie clear 

 evidence of a succession from comparatively basic lavas in the lower 



* Cule anrl Jennings, op. cit. p. 42G. 



