210 E. B. Tawney—Woodwardian Laboratory Notes. 
show stratiform planes in gabbros, etc., and I must relinquish at 
present any attempt at explanation. 
On the shore were loose blocks, one of which showed junction of 
the blacker olivine-diabase with the felspathic white-speckled diabase. 
I searched the ridge for a junction in place, and on the crest at one 
point was apparently a thick band of olivine rock lying on the 
lighter-coloured diabase ; I imagine this to be an oblique dyke 
inclosed in the other diabase, it had the same low angle of underlie 
as have the planes of the main mass of olivine-diabase. A sub- 
sidiary dyke might furnish proof of the relative ages of the two 
rocks—but in this case sufficient evidence was not found. 
The W. slopes of the ridge, where they come down to the shore, 
are covered with drift, cliffs of mud and transported blocks, except 
near the extreme point of the promontory washed by the sea, to which 
the state of the tide prevented access on foot, while the turbulence 
of the weather was against boating from Aberdaron. I fear the 
junction is covered up; it is possible, however, that there may be 
dykes in the end of the promontory or in the cliffs of the H. side, 
which might settle the relative age. 
Apart from lithological differences between the olivine- and other 
diabase,—one being greenish-black, the other brownish-grey spotted 
with white,—their appearance in the mass is by no means the same. 
They both have vertical joints, but the main feature of the olivine 
rock is a set of prominent planes dipping at a low angle to §. 30° H. 
or thereabout; the lighter-coloured diabase has oblique joints 
dipping about W. 20°S. and at a steeper angle. So that, from the 
strike of the joints and the form into which the exposures weather, 
one can tell at a glance whether an outcrop is olivine-diabase or not. 
At the N. end of Mynydd Penarfynydd the blocks which lie on 
the crest are not all actually in sité, so that the boundary cannot be 
laid down precisely ; they are mostly here diabase. Still the olivine- 
diabase at its N. termination must abut against the fossiliferous 
shales which extend from the farm below partly up the slope. 
Indeed, there is little doubt of its being intruded through them, for 
the shales which immediately 8. of Penarfynydd farm are black, 
soft, and fossiliferous, are seen 100 yards further 8.W. on the mid- 
slopes getting paler and harder, as if altered by baking. The N. 
end of the ridge which stands steeply almost immediately over the 
farm, and perhaps 100 feet above the fossiliferous shales, consists of 
pale-yellowish flaggy beds, with grey minute spots, apparently a 
further stage in the same process of change: they are here in 
contact with the diabase certainly, but perhaps also with the olivine- 
diabase, as the junction must be close. There is therefore, presum- 
ably, a double contact alteration, that of the crest certainly due in 
part to the diabase, and that on the W. mid-slope indicating a similar 
action on the part of the oiivine-diabase. 
If this be so, there need not be much difference in time between 
the two diabases, since they might both belong to the same epoch ; 
but I presume that the lighter-coloured diabase of Mynydd y Graig 
was the older, and that it was thoroughly consolidated before the 
