584 Chronicles of Science. [ Oct., 
and their illustration by Mr. Bone leaves nothing to be desired on 
that score. 
It is difficult to give in a short notice an adequate idea of the 
contents of such a work; it must therefore suffice to say that it 
consists chiefly of a detailed description of the Silurian rocks of 
North Wales, given in such a manner “that any one may ascertain 
the structure of any minor area in which he may be interested.” 
We shall therefore endeavour to cull only a few opinions and 
statements of the author, which may be of general interest. 
Of all subjects, perhaps that of breaks and unconformity is the 
one which Professor Ramsay has most successfully treated, not only 
in this work, but elsewhere. As regards the Lower Silurian rocks, 
there is no sign of unconformity between the Cambrian and the 
overlying Lingula-flags. The Tremadoc slates, which are ex- 
tremely local in their occurrence, and the fossils of which have an 
aspect of their own, though more like those of the Lingula-flags 
than of the overlying strata, are probably unconformable on the 
former. The Llandeilo-flags succeed the Tremadoc slates, and 
there seems good reason to suspect an unconformity here also; but 
between the Llandeilo-flags and the beds above (Bala or Caradoc) 
there is no sign of unconformity; nor has any been proved to 
exist between the Caradoc strata and the Lower Llandovery above. 
Here the Lower Silurian series ends. 
The Upper Silurian begins with the Upper Llandovery beds, 
which are found reposing indifferently and unconformably on any 
of the Lower Silurian deposits. This line of division, therefore, 
seems coincident with a natural break ; indeed, Professor Ramsay 
states that before the Upper Llandovery beds were formed, all the 
lower members of the Silurian series had been disturbed and planed 
across by denudation. The Tarannon Shale, which comes next, is 
sometimes conformable to the Upper Llandovery, but sometimes 
overlaps it, and reposes unconformably on older strata; but the 
Wenlock and Ludlow beds above are in conformable succession to it. 
Professor Ramsay’s opinion of the signification of these later 
unconformities is thus given :—“ The strata that lie between the 
top of the Caradoc, or Bala beds, and the base of the Wenlock 
Shale, were formed during a period of frequent oscillation of the 
relative level of the land to the sea. This oscillation succeeded the 
long and continuous deposition of the Bala beds, and preceded the 
formation of the Wenlock and Ludlow strata. The Lower and 
Upper Llandovery beds and the Tarannon shale belong, in fact, to 
a middle portion of the Silurian epoch only. Three fragments of 
this episode have alone been preserved, and while the oldest, that 
of the Lower Llandovery beds, is somewhat closely connected with 
the Lower Silurian period, the remaining two are more nearly 
related to the Upper Silurian age.” 
