In his “Geology of North Wales” (p. 102, 2nd ed.), Sir A. Ramsay figures a vertical cross-section of a greenstone dyke, which he describes as running along the cleavage-planes of the slate in the Ffestiniog quarries, and alternately “bulging and thinning off in a rapid succession of oval-shaped masses of 3 or 4 feet in length.” He seems to imply that this is one of the dykes posterior to the disturbance which produced the cleavage of the district. The ordinary post-Carboniferous dykes of North Wales, however, strike nearly at right angles to the cleavage; and further it is not easy to imagine any circumstances attending the intrusion of this one that would account for the phenomenon of alternate thickening and attenuation described. A precisely similar peculiarity is to be seen in ‘Dew's quarry’ at Pen-y-bryn, Nantlle.