188 
GENERAL VIEWS. 
ferent currents ; the greatest thickness and purity of the argillaceous 
deposits being to the west, and the same qualities belonging to the 
gritstones in the north, we may venture to suggest as an explanation 
the entrance of two distinct currents or primeval rivers, one on the 
west bearing sediment from the surface of a region of argillaceous 
slates, the other on the north bearing almost wholly the granular 
detritus of regions abounding in gneiss and mica slate ; both the 
deposits being modified by a continuous deposition of carbonate of 
lime. 
Where then shall we look for the surfaces of argillaceous schists 
thus raised above the sea, which might by wasting away yield materials 
for the shales of the Yoredale series of rocks? The Cumbrian slates 
might perhaps be imagined to have yielded, to eastward and southward 
currents, the argillaceous substances of the Yorkshire dales and hills of 
Derbyshire ; the Lammermuir to have supplied these of Northumber- 
land and Durham ; but whence have come the contemporaneous and 
very similar shales of the North-west of Ireland ? These appear to 
indicate more general agency ; perhaps to afford ground to believe that 
the real source of these materials was some more western land or shore 
now wasted away, or again submerged beneath the sea. 
The traces of the current from the north appear more and more 
distinctly as we proceed from Yorkshire through Aldstone moor to 
Northumberland : in the continual diminution of the finer and increase 
of the coarser varieties of gritstone. The gritstone rock below the 
underset limestone is in Yorkshire generally fine grained and useful for 
flagstone ; in Aldstone mooi’, under the name of Nattriss gill hazle, it 
is decidedly much coarser in grain, and more massive in structure ; 
further on in Northumberland it becomes a pebbly millstone grit, of 
greater importance in the sei’ies than any of the rocks of the true mill- 
stone grit series. 
If we follow this indication and look to the north for the local origin 
of the current which brought the arenaceous deposits of the mountain 
