﻿Vol. 66.] TKEMADOC SLATES OF SOUTH-EAST CARXAKVONSHIRE. 181 



Coming now to the jointing of the igneous rock-masses, we find 

 that those (the andesites) which are contemporaneous behave as 

 do the grits and grauwackes, and have joints in similar directions. 

 The early dolerites of Craig-ddu, etc. do likewise, but show some 

 evidence that when they were foliated by the cleavage-stress, they 

 were also sheared along certain pre-existing columnar joints. 

 This shearing is particularly evident at Gareg-goch, Careg-cnwc, 

 and at several points along the edge of the golf-links of Morfa- 

 bychan. 



The later gabbroid dolerites of Moel-y-gest, Craig-y-gesail, 

 Y Castell, and Pant Ifan, etc. behave very differently. Where not 

 too massive, as at Morfa Lodge, Portmadoc, they exhibit only 

 the rude columnar jointing which is characteristic of basic sills. 

 Where coarse-grained and very thick, they show similar columnar 

 structures in the fine-grained rock at their edges, but within the 

 main mass they break up more regularly along master-joints. 

 The most prominent of the master-joints are those which determine 

 the slab-like character of the cliff-faces, and these not following 

 any other rock-structure in the district range east and west and 

 have a steep hade to the south. On the bare hill- exposures, the 

 east-and-west joints frequently determine subsidiary crags, and 

 with dip-joints, occasional strike-joints normal to the sill surface, 

 and a platy structure which follows the elongation of felspar- 

 crystals, break up the gabbroid rock into blocks which are rudely 

 cuboidal. 



The hornstones immediately in contact with the gabbroid dolerites 

 have a system of jointing which is very complete, and partakes of 

 the arrangement of the columnar joints within the margin of the 

 dolerite. Completely recrystallized, they have lost all original 

 structures, except the chemical banding of their bedding which has 

 often become emphasized. 1 



The slabby rocks, ' desmoisites and spilosites/ 2 of the outer zone 

 of metamorphism have also a few east-and-west master-joints in 

 common with the altering rock; but, although they have now no 

 tendency to break along a cleavage, their most dominant 

 jointing always follows the cleavage-direction of the 

 country. This circumstance is suggestive, in that it helps to 

 confirm the post-cleavage age of the dolerites, and it is interesting 

 to note that the worm-tracks, nodular concretions, etc., which are 

 sometimes preserved upon the bedding-surfaces show evidence of 

 having been pulled out in a similar direction. The puckerings of 

 interstratified coarser bands and a certain ' nakiness ' of the 

 surface of the finer-grained slabs also show an elongation in this 

 same cleavage-direction. 



1 A. Harker, ' Bala Volcanic Series of Caernarvonshire ' (Sedgwick Prize 

 Essay) 1889, § vi. 



2 J. J. H. Teall, ' British Petrography ' 1888, p. 219. 



