66 £. B. Tawney—Woodwardian Laboratory Notes. 
felsitic character come in abruptly, and it is probable that these are 
of Arvonian age.” At the north end of the patch, by the Cromlech 
and Amwlch Lodge, the rock is however of the type which Dr. Hicks 
has termed Dimetian, and as it varies much in character at different 
parts of its course, I think we must ask for further proof that there 
is a second bed of rock of different age in this area. 
In fact the rock in the quarry by Amwlch Lodge, which the 
Geological Survey has here mapped as syenite, is somewhat like 
that of Twt Hill, Carnarvon ; it is a mixture of dirty white felspars, 
crystals, and equally large milky quartz, the two interlacing together, 
while in some of the interstices are specks of dark greenish-grey 
material. 
The contact with the dark grey Cambrian shales is an interesting 
one. It is not at all surprising from the character of the junction 
that the Geological Surveyors considered it an igneous rock. 
Section near Amwlch Lodge, Sarn. 
A Cambrian Shale. B Intrusive Dykes. C Metamorphic. 
In the accompanying diagram, compiled from two sides of the 
quarry, I have sought to represent the relative position of the beds. 
The shales seem in one place caught up between branches of the so- 
called syenite, since in the shale appears a projection of syenite. 
The shales are somewhat a paler grey near the junction, as if partly 
discoloured: this is not necessarily a sign of heat-action. There are 
two veins of decomposed trap-rock, one traversing the shales on the 
east side, the other ramifies in the so-called syenite. If this is not 
the contact of an igneous rock, it will be noticed at once that the 
steepness of the dip of the shales and absence of any shore deposit 
are against the notion that these deep-sea shales were deposited 
directly against an old Pre-Cambrian land. A fault would have to 
be inferred. There is, however, no clear line of junction. The 
contact is an irregular surface and the shales are joined on as tightly 
to the Dimetian as if they had been cemented by heat. Perhaps we 
may suppose that after the fault occurred further lateral pressure 
and movements had partly broken up the rocks at the junction and 
forced the hard rock as projecting processes into the shale. This 
would scarcely be synchronous with the intrusion of the igneous 
dykes, as lateral pressure would have tended to prevent or close any 
vertical fissures. Dr. Hicks’s specimens from this same area, which 
he terms Dimetian at Tymawr nearer Sarn, were described by Prof. 
Bonney [Q.J.G.S. vol. xxxv. p. 306]. 
