Alfred Harker—Local Thickening of Dykes. 69 
forms — surrounding included and narrower bands of Llandeilo 
rocks, and enveloped in turn by still broader sheets of Llandovery 
strata, etc. The great Gala (Tarannon) terrane must be coloured 
as the prevailing visible rock-mass of the Uplands, sweeping along 
the central and southern parts of the plateau in a broad sheet (from 
20 to 25 miles in width) from the North Channel to the German 
Ocean, only interrupted locally by long and narrow lenticles of pre- 
Gala rock, which decrease both in number and importance as we 
pass from south-west to north-east. Finally fringing the Gala 
Terrane along the south-east flanks of the Uplands from Burrow 
Head to the Cheviots will follow in natural superposition the slowly 
widening band of the Wenlock Ludlow (Riccarton) beds, the bounda- 
ries of which have been already sketched out by the officers of the 
Geological Survey. 
In the accompanying Plate (Plate III.) I have given some sketch- 
sections across the Upland region illustrative of my views of the 
general disposition and inter-relationships of its strata; and in the 
following “Table of Correlation” will be found incorporated some 
of the more important paleontological data which bear upon the 
question of the Upland Sequence. (See Folding Table II.) 
IV.—On Locat Tuickentne or Dykes anp Beps sy Foupine. 
By Atrrep Harker, M.A., F.G.S., 
Fellow of St. John’s College, Cambridge. 
N 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 aiternately “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. The same thing is 
figured by Lehmann (‘ Altkrystallinischen Schiefergesteine,’ pl. xiii. 
fie. 4) from granite-veins in the Saxon granulites. 
At several places in the Cambrian massif of Rocroi, notably north 
of Mairus near Monthermé, beds of grit intercalated among the 
slates are seen to swell and contract in the same manner. ‘Their 
thickness at the ventral segments is usually at least double of that 
at the nodes. The distance between the nodes seems to depend on 
the thickness of the bed. Gosselet has recently given a photograph 
of this ‘almond-like’ arrangement of the ‘quartzites’ in his great 
memoir on the Geology of the Ardenne. In these cases, at least, 
since we have to deal with sedimentary strata, it is clear that the 
