86 PEOCEEDINGS OF THE GEOLOGICAL SOCIETr. 



III. URICONIAN. 



Along the eastern borders of Wales a ridge of ancient rocks, 

 much broken by faults and presenting several striking unconforma- 

 bilities, has long been classic ground in geology from the descriptions 

 and illustrations of the " Silurian System." The main outlines of 

 the structure of that district, first admirably worked out by Mur- 

 chison, were delineated on the maps and sections of the Geological 

 Survey, wherein it was shown that in the Longmynd an enormously 

 thick group of stratified rocks, which, though unt'ossiliferous, were 

 referred to the Cambrian system, rose in the very heart of the 

 country ; that to the east of these rocks lay strata of Caradoc 

 or Eala age ; that by a great hiatus in the stratigraphy the 

 Upper- Silurian series transgressively wrapped round everything 

 below it ; that yet again the Coal-measures crept over all these 

 various Palseozoic formations, followed once more unconformably by 

 Permian and Triassic deposits. Besides all this evidence of extra- 

 ordinary and repeated terrestrial movement, it was found that the 

 region was traversed by some of the most powerful dislocations in 

 this country, while to complete the picture of disturbance many 

 protrusions of igneous rocks were recognized. 



In a territory so complicated, though it had been sedulously and 

 skilfully explored, there could hardly fail to remain features of 

 structure which had escaped the notice of the first observers. In 

 particular, the igneous rocks had been dealt with only in a general 

 way, and they consequently ofi'ered a favourable field for more de- 

 tailed study ; while, by a more searching examination of some of 

 the rocks for fossils, important corrections of the earlier work might 

 yet be made. 



A notable step towards a revision of the received opinions re- 

 garding the igneous rocks of this region was taken by Mr. AUport, 

 who showed that the so-called " greenstone " included masses of 

 devitrified spherulitic pitchstones and perlites, together with indu- 

 rated volcanic breccias, agglomerates, and ashes *. But the correc- 

 tion of the general view as to the geological age of these rocks is 

 due to Dr. Callaway, who, after spending much time and labour in 

 ascertaining by a careful search for fossils the position of the super- 

 incumbent rocks (wherein he discovered Cambrian organisms), and 

 in a detailed investigation of the structure and relationships of the 

 igneous masses themselves, was led to regard them as part of an 



* Quart. Journ. Geol. Soc. vol. xxxiii. (1877) p. 449. 



