420 
SILUKIA. 
[Chap. XVII. 
debted to the late Mr. Daniel Sharpe for nearly all they know of the real struc- 
ture and succession of the sedimentary rocks of the kingdom of Portugal, 
whether of Palaeozoic or Secondary age*. 
Of the former, two notable examples have been pointed out, and probably 
many more will be detected, in addition to another case to be presently noticed. 
The first noticed was at Vallongo, in the immediate environs of Oporto. There 
the Lower Silurian rocks, precisely of the same mineral type as those of France 
and Spain, and including the same fossils, together with two or three British 
forms and some new species, rise up in highly inclined and vertical strata ; and, 
as at Angers and other places, are quarried for roofing-slates. 
An anomalous arrangement of the lower and higher deposits, however, is there 
apparent. The coal-field of Vallongo, which has supplied a considerable portion 
of the city of Oporto with fuel, and in which are certain Plants not distinguish- 
able from those of the Carboniferous era, dips under the Lower Silurian schists 
with their characteristic Trilobites ! Now, if this had been really the normal 
position of the plant-bearing strata, we should have had to believe in the existence 
of an ante-Silurian flora composed of the same terrestrial vegetation as that 
which, in other regions, we find only when we approach to the horizon of the 
great coal-fields. Believing fully in the accuracy of Mr. Sharpe's sections, I 
saw, -however, from his own faithful description of this district and the region 
to the south of it, that we might without difficulty surmise how this apparent 
anomaly has been brought about. The Lower Silurian rocks and the contiguous 
Coal-strata (for the coal is not found within the body of the lower slates) are 
both situated between two ranges of eruptive rocks, — the one on which Oporto 
stands being granite, and the other, to the east, syenite. On the flanks of the 
granite of Oporto, micaceous schists abound, which, if metamorphic, may be of 
any age ; but, even if these be of ante-Silurian date, we have simply to imagine 
a trough of coal, of the true Carboniferous date, placed between these schists, 
on the one hand, and the clay- slates with Lower Silurian fossils, on the other, 
and then, by a movement of which we have many well-authenticated examples 
both in Europe and America, that this trough has been placed locally in an in- 
verted and dislocated position. 
The enormous length of time which must have elapsed between the accumu- 
lation of the Lower Silurian and the formation of the Devonian rocks, and during 
which interval we have here no evidence of Land Plants having appeared, forbade 
us, indeed, to adopt the view of an infra-Silurian coal, until we had exhausted every 
other means of explaining the anomaly ; for the plications of the strata in Belgium, 
as delineated by M. Dumont, or those in Westphalia, mentioned in this volume (p. 
403), or those in the Alleghanies (described by Prof. Rogers), explain how strata 
really inverted in one place, may be followed until they resume their regular order. 
Wedged in, as these Silurian and Carboniferous masses of Portugal are, be- 
tween two flanking parallel ridges of eruptive rock, it was no doubt difficult to 
detect their regular order — though, even in describing a transverse section from 
Oporto to Aveiro, Mr. Sharpe himself stated that clay-slates, lying on gneissose 
and micaceous schists, are surmounted by carbonaceous shale and red sandstone. 
This I conceived to be the natural order, because even in the adjacent districts of 
the north of Spain, to which that geologist first pointed attention as being likely 
to contain Silurian rocks, there are no traces whatever of anthracitic coal-mea- 
sures beneath or associated with such ancient rocks, all the Carboniferous depo- 
sits of that region being, as previously stated, in their usual and normal position. 
* See Quart. Journ. Geol. Soc. Lond. vol. v. p. 142; vol. vi. pp. 101 & 135; vol. ix. p. 146. 
