THE VALE OF CLWYD. 9 
alternations of sandstone with the mudstone, showing that 
there had been changes somewhere near, exposing land to more 
rapid denudation and permitting the existence of currents which 
could carry much coarser sediment over our area. [See Q.J.G.S., 
Vol. xxxiii., May 1877, p- 207.] This is confirmed by palzon- 
tology; for these changes, whatever they may have been, seem 
to have unsettled the conditions upon which life depends, not 
only in our district, but all over Europe, and so we find at this 
horizon a new fauna commencing. We must notice another 
renfarkable fact with regard to the Silurian. In South and 
Central Wales we have a different type from that which prevails 
in North Wales—so much so that we cannot correlate the 
subdivisions—but of this I will speak again when I come to 
the details. 
The sandy Mudstones are generally of a pale colour, and are 
associated with fine pasty rock known as the Pale Slates. 
Sometimes we find in these mudstones and slates thin bands 
of black shale, with graptolites. The species I have found 
are Monograptus convolutus, (His), M. triangularis, ( Harkn.)/, 
M. tenuis, (Portl.), M. Sp. The beds probably represent in part 
fhe graptolithic mudstone of the Lake District. I think that 
zones could be locally distinguished, and many of them corre- 
lated with those of other areas. This stage passes up occasion- 
ally through beds of wavy banded sandstone. into flaggy shales 
with subordinate beds of sandstone, into the main mass of the 
Denbigh Grits and_ Flags. It is exposed here and there on 
Bryngorlan, at the South end of the Vale of Clwyd, and may be 
well examined in the gorge of the Clwyd under Dinas, W. of 
Derwen Station, and between the valleys of the Clwyd and Dee 
(see figs. 1 & 2.) [See Q.J.G.S., Vol. xxxv, Nov. 1879, p- 697-] I 
would here offer an explanation as to the use of the term grit. A 
geologist who had been working in the Carboniferous rocks would 
be puzzled if he searched for grit in the Silurian of our district. 
His idea of a grit would be the Millstone Grit, but among the 
Silurian and Cambrian rocks the term is applied to any sandstone 
which has been consolidated into a tough massive rock, showing 
no cleavage, seldom splitting along the bedding and feeling 
rough to the hand. The grains of which it is composed are 
generally much smaller than those which make up the sandstones 
of the Carboniferous rocks. It is in fact a sandstone being 
changed to quartzite, and it has been proposed to use the old 
word Grauwacke as a lithological term for this class of rock. 
Bare) Mag., Vol. iv., pp. 229—355, footnotes.] Very likely 
rauwacke will be required as a stratigraphical term to include 
everything from the base of the Cambrian to the top of the 
Silurian, and perhaps all the beds conformable to them. So it 
will. be better for the present to use the term sandstone instead 
of grit for these Silurian deposits, except in the case of a few 
thin beds in which the grains are so large as to fairly entitle the 
rock material to the name of grit, as for instance among the 
