816 REPORT—1890. 
accounted for by the dissociation of the water and of the chloride of sodium by 
the intense heat,' and the combination of the two gases thus formed would result in 
the production of hydrochloric acid. 
In the case of the cooler granites there is no question of dissociation and of 
gases, but of the entanglement of brine and steam at more moderate temperatures. 
Thus the access of salt water to highly heated rocks seems to account for some 
of the more important gases emitted by lavas and of the more characteristic fluid 
inclusions caught up by granites. 
An alternative theory, that the crystals of salt in the Dartmoor rocks ‘had 
been formed from hydrochloric acid acting on the soda in the rocks,’ does not 
seem to the author to account for the crystals in the quartz-veins of the culm slates, 
or to explain the complete permeation of the granite by the chloride of sodium. 
Moreover, the one theory accounts for the presence and origin of the hydrochloric 
acid as well as of the soda, whereas the other has to assume the previous existence 
of soda and the advent of hydrochloric acid from unknown quarters. 
6. On the Strata forming the Base of the Silurian in North-East 
Montgomeryshire. By J. Bickerton Moraan, F.G.S. 
The area in which the rocks referred to in this communication occur is situate 
between the towns of Welshpool and Llanfyllin, on the North Wales border. 
These basal rocks, which were investigated by the author at the suggestion of Pro- 
fessor Lapworth, are first seen in Powis Castle Park, one mile to the south-west 
of the former place, where they come to the surface in the form of a small anticline,. 
the southern limb of which furnishes the foundation upon which stands the ancient 
and picturesque structure of Powis Castle. From this point they take a north-: 
easterly direction, and, passing through the upper, or western, portion of the 
town, are abruptly terminated at Red Bank by a north-east and south-west fault.. 
Westward of the town they crop out in the neighbourhood of Frochas, and, 
striking thence through the folded strata north-eastward, they extend for several 
miles in the direction of Llansaintffraid. 
The character of these basement rocks is, for the most part, that of a hard 
quartzose grit, the base of which, in places, takes the form of a coarse purple 
conglomerate, and which sometimes includes amongst its more siliceous constituents 
pebbles obtained from the underlying rocks, and occasionally contains green, 
earthy, concretionary patches. The grit-beds are sometimes sub-calcareous, and 
graduate upwards into fine-grained sandstones, the whole being characterised by 
possessing a deep red colour. 
On sheet 60 N.E. of the Geological Survey Map these grits and sandstones are 
shown as Caradoc, and in both ‘The Silurian System’ and ‘Siluria’ Sir Roderick 
Murchison identifies them as belonging to the upper portion of his Caradoc sand- 
stone. Although fossils are by no means abundant or generally distributed, 
sufficient palxontological evidence has been obtained from these beds to prove 
that they are of unquestionable May Hill age. 
As these strata are followed from point to point in the district, they are found 
to repose transgressively upon different zones of the underlying Ordovician Rocks, 
so that in this area there is a distinct prolongation of the regional unconformity 
between the Ordovician and Silurian systems, an unconformity which can now be: 
followed continuously from Llandeilo to Llanfyllin. 
Above these red rocks comes a series of shales, mudstones, and sandstones, in 
which occur occasional courses of more calcareous matter, containing fossils of 
Lower Wenlock age. 
The discovery of the May Hill age of these rocks will, therefore, necessitate a 
re-mapping of the district for the purpose of rectifying the boundary line at the 
base of the Silurian—a task the author hopes to complete in his leisure time. 
' See Characteristics of Volcanoes, J. D. Dana, p. 8. 
ia ——s 
