450 ME. A. HABKER ON THE EEITPTIVE EOCKS IN THE 



coidal cleavage. The well-known " hour-glass " structure is not 

 uncommon. The mineral is either in ophitic plates or in grains 

 between the felspars. Only occasionallj'', as at Castell Carron, does 

 it appear with idiomorphic boundaries ; sometimes it shows twinning 

 on the orthopinacoid. 



The structure of these diabases is rather variable. Sometimes 

 they are ophitic, but the augite-plates are never of great size, and 

 the more usual texture rather approaches the grauulitic type. It 

 does not appear that the basic rocks of this area bear out any rule 

 associating the ophitic structure with the larger masses, and the 

 granulitic with small dykes and sheets. The types of structure 

 seem rather to be connected with the relative proportions of the two 

 minerals^ the ophitic rocks being those which have the most abundant 

 augite. 



These diabases afford fine examples of spheroidal jointing and the 

 consequent weathering in concentric crusts. The spheroids vary in 

 diameter from two or three inches to as many feet ; often a cluster 

 of small ones is included in one large shell. The spheroids are 

 sometimes formed in the interspaces marked out by plane joints, but 

 the most regular examples often have no such relation. The great 

 mass of Mynydd-y-Ehiw, especially in its northern and southern 

 portions, is largely built up of spheroidally jointed rock. 



The contact-effects of the diabase upon the adjacent strata usually 

 present no special features. In one case the strata of indurated 

 shale show a curious modification. They are broken by joints 

 parallel and perpendicular to the bedding into small rectangular 

 blocks, in each of which lies an ellipsoidal nucleus. The ellipsoidal 

 joints are from an inch or two to a foot in diameter. This is at the 

 contact with a dyke 35 yards wide, about a quarter of a mile west of 

 Sarn. 



The relations of the diabase to the other intrusive rocks can 

 nowhere be demonstrated by actual sections, and the age of the mass 

 is a matter for conjecture. It can only be said that the diabase cuts 

 through Arenig or Lower Bala strata, while no rock of similar type 

 is certainly known in Caernarvonshire of later age than the Eala. 

 The presumption is therefore in favour of assigning the intrusion of 

 the rock to the Bala age. 



V. The Hoenblende -Diabase. 



Eocks of this family are largely developed in the southern part 

 of the district. They are seen everywhere on Mynydd Penarfynydd, 

 with the exception of the outcrop of hornblende-picrite on the west 

 and south-west slopes. They form the whole of Mynydd-y-graig 

 and its outlying spurs, the extent from south-west to north-east 

 being about 1|- mile. Hornblende-diabase may occur under the low 

 ground north-west of Mynydd-y-graig, but probably the diabase 

 extends as far as this. The same rock is found in force in the 

 neighbourhood of Rhiw : it occurs a few hundred yards north of the 



