SILURIAN ROCKS OF NORTH WALES. 143 
In the southern area the basement beds are greatly thickened 
by the development of the May Hill Series, and the upper- 
most beds pass gradually up into an enormous mass of deposits 
which have been erroneously included in the Old Red Sandstone. 
In the northern sections, on the other hand, there is none of 
this covering deposit of red rocks, nor are the highest beds of 
the Silurian itself generally to be identified, but the basement 
_ bed of the Carboniferous is superposed unconformably and 
transgressively on all the various members of the Silurian 
Series. Nor is it only in the incoming and outgoing of the series 
that this difference between the southern and northern types is 
seen. Inthe middle, also, there are the same indications of 
some difference in the conditions affecting sedimentation, for 
there are in the southern area well marked limestones occurring 
at fairly constant horizons which were of course seized upon 
as convenient for purposes of classification and correlation, 
though they do not always coincide with the most marked 
paleontological or other natural divisions. In the northern area, 
on the other hand, these limestones are absent, or only to be 
tentatively identified with certain calcareous bands caused by 
banks of shells which, though ranging above and below, happen 
to be there so much more numerous as to have given rise to 
bands of limestone. 
In the northern area there are enormous masses of sandstone, 
generally of very uniform texture. These beds have, to the 
touch, all the roughness of a coarser rock, and, contrasting 
strongly with the shales among which they occur, were first 
spoken of as Grits, as in the case of Denbigh Grit, Coniston 
Grit, &c., aterm which should be reserved for a rock of much 
larger grain. There are, however, some lenticular beds among 
them, which may properly be spoken of as Grits, where the 
grains are as coarse as mustard seed or sweet pea. In the 
southern area there is very little sandstone or grit. The great 
thickness of the basement series, the prevalence of finer 
sediment, and the recurrence of limestones throughout in the 
southern sections probably point to a more rapid “sinking of 
the sea bottom in that area, so that by the time that the middle 
and upper part of the deposit had been laid down, shallow water 
conditions could only prevail where the whole of that part of 
the sea had been silted up Then, when the sea bottom was 
within reach of wind waves, and of strong running tidal action, 
coarse sands would be again carried over the area from a 
distance, and spread here and there above the thousands of feet 
of muddy sediment, the deposit of which had kept pace with 
the depression of the sea bottom. 
Perhaps the most convenient mode of laying the matter before 
the Society will be to describe the characteristics of each section 
of the table separately. 
SecTion A. 
In Carmarthenshire the section seen up the River Sawdde, 
near Llangadock, gives the best succession for establishing the 
