Br. R. D. Roberts — On the Geology of Anglesey. 



153 



glomerate is the basement bed of the Cambrian resting against the 

 Pre-Cambrian granitoid basis. 



2. I also identify the quartz conglomerate at Tvvt Hill with, that 

 at Nebo. 



3. I hold that the disagreement in dip of the conglomerate and 

 shales, in the northern quarry at Nebo, is due to a fault, as shown 

 in Fig. 3 in my paper (Geol. Mag. 1881, p. 197). The discord- 

 ancy between tlie shales and conglomerate in the other or lower 

 quarry of Dr. Callaway is due to the crushing of the conformable 

 overlying shale during the movements of the underlying unyielding 

 grits, but the apparent unconformity indicated by Dr. Callaway in 

 his section (Q. J. G. S. vol. xxxvii. 1881, pi. viii. fig. 6) does not 

 exist. The divisional planes which he takes for bedding are 

 joints approximately parallel to the fault. To make this clear I 

 give a copy of Dr. Callaway's section, Fig. 1, and my own 

 section of the same quarry, Fig. 2. 



Fig. 1. Section across Lower (Southern) Quarry, S. of Nebo, Anglesey. 

 Dr. Callaway's Section (Q. G. J. S. vol. xxxvii. pi. viii. fig. 6*). 

 N.W. ^___-^r- — _ S.E. 



a = Cambrian shales 

 Fiff. 2 



5 = Cambrian grit (called by Dr. Callaway Pre-Cambrian). 



Quarry. 



Section across lower (Southern) Quarry, S. of Nebo, Anglesey. 

 Dr. Eoberts's Section. 



Quarry. 



F F 



a. Black slate much crushed = Cambrian shales. 



5 = Cambrian grit (called by Dr. Callaway Pre-Cambrian). 



b'. Black slate and subordinate sandstone. 



c. Veined and jointed grits, conglomeratic in parts. 



(^ = Dyke. J' J?= Faults. 



The points at issue must now, I think, be perfectly clear. 



Dr. Callaway (Geol. Mag. Feb. 1882, p. 57) says, referring to my 

 criticism, "I have always been perfectly aware that the black shale 

 is faulted against the grit. Indeed, in my note-book I find a section 

 (dated July 20, 1880) which is almost a facsimile of the figure in 

 Dr. Eoberts's paper." He then adds that his view is that the shales 

 were both unconformable and faulted. I have quoted in an earlier 



* In Dr. Callaway's Plate, the upper bed a is marked " c = Cambrian ? " ; and the 

 lower bed is marked " a = Gneissic series," " fig. 6, c on a." 



