Reviews — Ramsay's Geology of North Wales. 559 



The beautiful maps, of which this memoir is descriptive, have long 

 testified to the great labour and the admirable skill employed in 

 their construction. The careful delineation of the chief rock- 

 divisions in a profoundly faulted and contorted region must always 

 be tedious and difficult, but when, in addition to this, there are 

 innumerable beds and masses, of volcanic and metamorphic origin, — 

 many of them of singularly erratic character, — to be defined upon the 

 map, perhaps only those who have had a like task to perform can 

 appreciate the degree of mental and bodily fatigue involved. As an 

 example of the skill with which the disjointed strata have as it were 

 been pieced together, we might instance the recognition of the 

 felstones and ashes of the wild Snowdonian district as the equivalents 

 of the Bala limestone beds. The numerous sections and copious 

 details supplied in the memoir in regard to this point, amply bear 

 out the conclusions expressed on the large " six-inch " sections of the 

 Survey, which have now been before the public for some years. 



Those who were interested in Prof. Ramsay's Presidential Address 

 to the Geological Society in 1863, will be glad to have the details 

 here given of the stratigraphical breaks that characterize the Silurian 

 formation. The reasons for inferring (apart from fossil evidence) 

 that the Llandeilo and Bala beds over-lap unconformably the Lingula 

 flags are very clearly stated, and the other breaks in succession are 

 further elucidated. 



The igneous geology of North Wales occupies, as might be 

 expected, a considerable portion of the memoir, and those who work 

 at this department of the science will find here a large accumulation 

 of facts, and among them, some rather hard problems to solve, Two 

 great groups of contemporaneous felstones and ashes are described, 

 the .first of which forms the basement of the Llandeilo beds, and the 

 second occupies the horizon of the Bala limestone. From the great 

 bulk attained by these rocks in Cader Idris and Aran Mowddwy, and 

 from the fact that, when traced westward, they are found to decrease 

 rapidly in thickness, it is inferred that the volcanic centre or centres 

 were probably to the eastward of a line drawn between Tremadoc on 

 the north, and Llanegryn on the south; and it is further conjectured 

 that some of the felspathic masses that break through the Silurians, 

 in the region indicated, may possibly mark the site of the volcanic 

 foci. The innumerable intrusive greenstones that lie either among 

 or beneath the bedded traps and ashes, but which never make their 

 way through the overlying strata, so as to assume the character of 

 lava-flows, are a great puzzle. Their position proves them to be 

 "intimately connected with the volcanic phenomena of the district," 

 but the nature of that connexion cannot be clearly made out. The 

 almost total absence of hornblende from the felstones renders it more 

 than improbable, as Professor Ramsay remarks, that they could ever 

 have formed the upper or lava-portion of a melted mass, which at 

 certain depths fvom the surface crystallized into greenstone. The 

 greenstones are therefore not likely to have been the feeders of any 

 of the overlying felspathic rocks : unless, indeed, we hazard the 

 speculation that a melted mass (containing the elements of horn- 



