508 REPORT— 1883. 
have been recently asserted by Dr. Geikie to be partly Cambrian and partly 
intrusive. The author contends that they are Precambrian in age, but form a 
very complete volcanic series, which may well be designated the Dimetian. 
The basis of the series is the Dimetian granite, serving as the core. This is 
surrounded by the more acid rocks, as the quartz-felsites and felspar porphyries 
(the so-called Arvonian), and the more outlying portions consist of very varying 
materials, chiefly rough ashes or agglomerate breccias—on the east side finely 
bedded ‘ halleflintas,’ and on the north side many basic lava flows. These are the 
so-called ‘Pebdian.’ The arrangement of these rocks shows the characteristic 
irregularity of volcanic rocks, and though many portions are bedded, they have no 
dominant strike over the whole district. The Cambrian series commencing with 
the conglomerates is quite independent and hangs together asa whole. In no 
place can a continuous passage be proved from the one series to the other ; the 
junction is in most cases a faulted one, and at the places where this is not so, the 
conglomerate lies on different beds of the volcanic series. 
The proofs of the Precambrian age of the volcanic series may be seen (1) 
between Nun’s Chapel and Caerbwdy, where the junction is faulted towards the 
west, so that more and more of the series above the felspar-porphyries comes in 
towards the east, and the conglomerate contains fragments of it at Caerbwdy ; 
(2) at Porthclais, and Ogof Llesugn, where the junctions are faulted, but the 
conglomerates hang to the Cambrian beds; (3) at Penmaen Melgn, where the con- 
glomerate lies apparently without a fault, on the ashes and agglomerates which 
are not schistose; (4) at Castell, where the junction is faulted, the Cambrian 
striking at the volcanic series; (5) in Ramsey Island, where fragments of Arenig 
rocks, and conglomerate (of peculiar character) are equally let down amongst the 
ashes; (6) along the northern boundary of the mass, where different members of 
the Cambrian series come in contact with it. 
The granite is nowhere intrusive, or in any way connected with the Cambrian 
rocks. It cannot even be proved intrusive in the volcanic series. The section at 
Ogof Llesugn shows a double fault of great magnitude forming the boundary of 
the granite as far as Porthclais, with a mass of conglomerate wedged in between 
the two faults, both of which are slickensided. The apparent welding of the 
conglomerate to the granite is due to the intrusion of the diabase along the fault 
which has caught up portions of each. This is the only place (except at Porth- 
seli, where it has become schistose, perhaps by faulting) where there is any notable 
alteration in the quartz-conglomerate. The other junctions, whether at the faults 
or at the boundaries of the crystalline rocks, show little or no change. There is no 
proof of an isocline west of the granitic mass, but of a very variable series of 
ashes and lavas, with interstratified calcareous bands. None of the crystalline 
portions of the series show any signs of true bedding. The Cambrian beds which 
can be compared to tuffs, though they have not been proved to be such, are far 
away from the base of the series, and bear no relation to the underlying ashes. 
Similar phenomena to these are repeated further to the east, in the localities 
pointed out by Dr. Hicks. Hence in these ancient times there was a tendency to a 
linear arrangement of volcanic outbursts—the central and older portions are more 
crystalline and acid, while the ashes and more basic flows, with the stratified 
siliceous tufts, form the outworks round each centre. On the possibly denuded sur- 
face of these rocks the Cambrian beds were deposited, the conglomerates deriving at 
least their matrix from them; and at a later date the vertical direction was given 
to them by the forcing up of the great volcanic series to their level, in some places 
the granitic base, at others only the ashy surface. At this, or at some later 
period, the diabase dykes invaded both. 
4. On the Geology of the Troad. By J. S. Dittzr. 
This paper gave a brief account of recent researches in the Troad, the author 
being attached as geologist to the United States Assos Expedition. Further 
details, especially as regards the igneous rocks, were submitted to the Geological 
