THE GLACIAL DEPOSITS OF NORTHERN PEMBROKESHIRE. 79 
The writer discovered a big boulder of picrite on Strumble Head, on a piece of 
uncultivated ground a little north of Tre-sinwen farm, by the side of the pathway 
leading to the coastguard station at the Head. The boulder had been broken, and 
now lies in several pieces—the biggest piece measuring roughly 34x 3x2 feet—and 
in its original condition it must have been much bigger. Another boulder of picrite, 
very similar in appearance and size, and also broken, was discovered on a field lying a. 
little to the north-west of Tre-seissyllt, between the farm and the coast north of Aber- 
bach. A microscopic section of this rock revealed the presence of the following 
minerals :—brown hornblende, strongly dichroic; augite, nearly colourless ; olivine, 
not very abundant, and always very much serpentinised and of a greenish colour ; 
magnetite ; a chloritic mineral, which is evidently an alteration product, and a little 
plagioclase felspar. The chloritic mineral is greenish in colour and markedly dichroic. 
The general appearance of the rock as seen in section was very different to that of 
the St David’s picrite: the augite does not show such perfect forms in the former 
as in the latter, and there is rather more olivine in the St David’s rock. The 
Strumble Head picrite boulder is rather more like some of the Penarfynydd speci- 
mens as seen in section, but there is not so much olivine in the former, and the 
peecilitic structure which often characterises the latter is not seen in the former. 
But it is highly probable that the Strumble Head picrites have also been borne 
from Lleyn or Anglesea. The two specimens found on Strumble Head le about 
three miles apart, in a line whose direction is north-north-east to south-south-west. 
Near the Tre-seissyllt boulder of picrite lay a boulder of olivine-gabbro, also broken 
to pieces by blasting. The newly-exposed faces were remarkably striking, and the 
erystals are very fresh. The rock is quite unlike the gabbros found in Pembroke- 
shire. A microscopic section showed beautifully fresh olivine crystals, and the rock 
is undoubtedly of Tertiary age, and has probably come either from the Western Isles 
of Scotland or from the north-east of Ireland. 
2. Hrraties in the Drift.—As might be expected, the majority of the boulders. 
found in the drift deposits are of local origin. They occur abundantly in the Upper 
Boulder-Clay and Rubbly-Drift, and in the sands and gravels, and to a less extent 
in the Lower Boulder-Clay. The grits, shales, and slaty rocks of Pembrokeshire are 
_ very similar in appearance to rocks of a like nature from North Wales, and the same 
is true of some of the igneous rocks, especially the diabases and some of the lavas. 
It is thus quite possible that among the boulders found imbedded in the drift many 
North Wales rocks may be represented, though there is no means of distinguishing 
them readily from the boulders of local origin. This was suggested to the writer by 
the discovery of boulders of a diabase rock in the boulder-clay exposed at Cardigan. 
To the naked eye this diabase seemed very like that found to the south-west in 
Pembrokeshire. But no such rock is known to occur anywhere nearer Cardigan than 
Newport—nine or ten miles to the south-west—and so these boulders must have come 
from the north. This is made all the more probable by the discovery of boulders of 
