360 Mr. Farey’s Notes on Mr. Bakewell’s Geology. 
P. 108. which range about WYN IV and ESE, The most 
considerable of these Dykes, seems to intersect 
and cross all the Coal-measures, to the westward 
of Pentre-Berw above mentioned, to cut through 
the Limestone Rocks, which form the floor or 
trough in which the Coal-measures rest (see my 
Notes on pages 275 and 985) and to penetrate 
the vast series of coarse Slate beneath all these, 
along the course of Pentre-Berw Brook ; and pro- 
ceeding thence nearly in a straight line eastward, 
‘for Gairwan yicha and the S side of Plas Newydd 
House, on the banks of the Menai straight, as I 
have been informed. 
Wherever I have had the opportunity of ex- 
amining this Dyke, it has appeared to be neat 40 
yards wide, and the Basalt to be principally in a 
decomposed state, like very coarse brown grey 
‘ Joamy Sand, but so compact, that it can with dif- 
ficulty be cut with a spade, in the several ‘¢ Gravel- 
pits,” as they aré there called, which are opened 
in it, for procuring Sand to mix with Lime, which 
it occasions to set very hard in the mortar of walls 
built therewith; many Balls and large ‘irregular 
Blocks of black and very hard Basalt (called Lron- 
stone by the inhabitants) are seen lodged in or 
detached from this dyke, below Pentre- Berw 
Mill. 
The other large Dyke, is filled with this solid and 
durable Basalt ; it intersects and crosses the Coal- 
basin, about 23m. southward of the iast, and pass- 
ing close on the south side of Pencrug Colliery 
(1m. E of Trefdraeth Church), it appeared to oc- 
casion the Coal-seam therein, to be much harder 
and less inflammable (though not by ‘‘ charring,” 
T believe, see Note on p. 125), than in the other 
Collieries on the same seam to the south-west of 
this place, and whence this Pencrug Coal was 
called Cudém, and sold for the same uses, as the 
small of theStone-coal brought from South Wales, 
under that appellation, On the N side of Ty- 
Calch Lime Works, this dyke intersects the Lime- 
stone Rock; and in its course westward, soon 
after it has entered the coarse Slate, tar. WNW 
of Ty Calch House, a large mass or rock of white 
Marble stands up in its range, above the sur- 
rounding Slate, and which in its quality, seemed 
; , to 
5 
