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LOWER PALHOZOIC ROCKS OF ANGLESEY. 579 
none are found. Indeed, I have not been able to discover this schist 
in any Anglesey conglomerate, most of the contained fragments 
being either gneissic or slaty. This is a very singular result if the 
schist is intermediate in age between the granitoidite and the slaty 
veins; but is perfectly intelligible on the supposition that the 
Holyhead schist is older than the granitoid rock, and had not been 
exposed by denudation when the conglomerate was formed. 
From the ground just described we have clearly ascertained the 
following facts :— 
1. Both the granitoid and the slaty rock of Tywyn are older than 
the Llanfihangel conglomerate. 
2. The granitoid is older than the slaty rock. The presence in 
Anglesey of two formations anterior in age to the Llanfihangel 
conglomerate is thus distinctly proved. 
D. AGE oF THE oLpDEST Patmozorc Rocks. 
The problem of the age of the altered rocks of Anglesey appeared 
to be simplified by the alleged discovery of Tremadoc fossils in the 
lower Paleozoic strata. The most prominent of these fossils is an 
Orthis, which has been identified as O. Carauswi. It occurs in great 
numbers in the grit which overlies the basement conglomerate, and. 
it may be traced from Llanfaelog, near Cymmeran Bay, to the 
northern Bryn-gwallen, north-west of Llanerchymedd. ‘The last 
locality requires special notice. 
About half a mile to the south, near another Bryn-gwallen, the 
Orthis-band lies a little above a quartzose conglomerate, about 
a hundred yards from an inlier of the granitoid Archean, as 
noticed by Prof. Hughes and Dr. Roberts. At the northern locality 
the facts are similar. In a field near Tyn-y-buarth, is another 
inlier of the gneiss, with conglomerate resting on it to the west ; 
and at a short distance, just inside the next field, is a quarry of 
the grit, containing fragments of what appear to be Lllenus* and 
Asaphus*. In a similar grit in the neighbouring walls we find the 
Asaphus, with Ogygia* and the Orthis. This band is certainly on 
the horizon of the Orthis-grit of the southern localities; for, in 
addition to the identity of the Orthis, the rock is at about the same 
distance above the gneiss as in the section at the southern Bryn- 
ewallen. 
As the Orthis occurs in the same beds with what seem to be 
typical Ordovician genera, it may appear superfluous to discuss its 
affinities ; but I venture to think that, without the Trilobites, 
the Brachiopod would not be decisive of the question. This species 
is supposed to be separated from O. calligramma by its smaller size, 
its fewer ribs, and the comparative flatness of its dorsal valve. But 
O. calligramma is a very variable form. I have collected, from 
undoubted Ordovician strata in North Wales and North America, 
* These Trilobites have been named for me by Dr. H. Woodward, F.R.S., 
but he states that their fragmentary character renders their identification a 
matter of difficulty. 
