Vol. 62.] ORDOVICIAN OP WESTERN CARMARTHENSHIRE. 601 



great deal, so much so in fact, that only an imperfect idea of the 

 actual conditions can be gleaned from what can be represented 

 on a 1-inch map. 



North of the main axis, a line of extensive disturbance commences 

 near Castell-gorfod, and runs slightly north of west, past Penlan 

 and Nant-yr-eglwys to Llanboidy, and thence to Llanglydwen, the 

 structure of the ground increasing in complexity as it passes 

 westward. Some of the ground near Penlan, INTant-yr-eglwys, and 

 south of Eglwys-fair-a-churig is very obscure and difficult of 

 interpretation. 



(3) Cleavage. — The rocks in the northern limb of the fold are 

 more or less affected by cleavage. In the lower beds it is slight, but 

 very pronounced in the upper, increasing in intensity as it is traced 

 northwards. Thus the Didymograptus-bifidus Beds are only slightly 

 affected, the D.-Murchisoni Beds more so, and the Dicranograptus- 

 and higher beds so much at some points as to become quite slaty. 



The faulting was anterior to the cleavage, for the latter is quite 

 independent of the former. Dips vary a great deal both in 

 inclination and in direction, but the cleavage-planes are tolerably 

 uniform and constant, running east and west with an inclination of 

 about 45° north, or, occasionally, a little east of north. 



On the south there is an almost complete absence of cleavage ; 

 only in one or two localities— in black DicranograptusShsdes — can 

 its occurrence be suspected. 



(4) Igneous Hocks. — North of Llanboidy and extending from 

 a point about a quarter of a mile west of Maesgwynne House to 

 the Fenni Valley, near Maencoch, there is an intrusion of diabase. 

 This ' dyke ' crosses outcrops of Didymograptus-bifidus Beds, Asaplius- 

 tyrannus Beds, and, for some distan ce, penetrates into Dicranograptus- 

 Shales. It also crosses at least one of the main faults, without 

 appearing to be in the least displaced by it. In some places, the 

 rocks in contact with the dyke show evidence of slight baking, 

 but the portion affected is only a few inches thick. 



At Glyntaf House, a mile south of Llanglydwen, are two exposures 

 of diabase extending for a short distance east of the Tave, the bed of 

 which is here much encumbered by huge masses of the rock. The 

 northernmost is seen in section in a railway-cutting near the 

 round cottage, as well as in the old quarry behind the cottage itself. 

 There it is seen to cut obliquely through cleaved ash of a greyish- 

 green colour. The other exposure also abuts on the railway, but 

 about a quarter of a mile* farther south. This dyke has been 

 somewhat extensively quarried immediately west of Glyntaf, but 

 seems to die out at no great distance to the east. West of the Tave 

 both dykes are much more developed, and seem to be an eastward 

 extension of the volcanic rocks of the Prescelly country farther west. 



(5) Contour, etc. — The district is drained by the Eiver Tave 

 and its feeders, which have hollowed out for themselves numerous 

 deep valleys and dingles. The general direction of these valleys 



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