222 
THE SILURIAN VOLCANOES 
BOOK IV 
referred to. These may quite well exist; but there is assuredly no one 
gigantic displacement, such as the theory I am combating would require ; 
while any faults which do occur cannot be greatly different from the others 
of the district, and do not prevent the true relations of the rocks from beino- 
discoverable. “ 
Where the supposed elliptical fault reaches the shore at Carmel Point, 
the two groups of rock seem to me to follow each other in unbroken 
sequence.^ The black slates, which are admittedly Lower Silurian, dip 
underneath a breccia and greenish (Amlwch) slates. Not only so,’ but 
bands of similar lilack slates occur higher up, interstratified with’ and 
shading-off into tuffs and greenish slates. Further, bands of coarse vol- 
canic breccia occur among the black slates south of the supposed break 
These, in accordance with the exigencies of theory, are represented as 
separated by a network of faults from the black slates amid which they lie. 
But good evidence may be found that they are truly interbedded in these 
slates. In short, the whole of the rocks in that part of Anglesey form one 
great series, consisting partly of black slates, partly of greenish slates, with 
abundant intercalations of volcanic detritus. The age of the base of this 
series is moreover determined by the occurrence of Bala fossils in a band of 
limestone near Carmel Point. 
The rocks which extend eastward along the coast from the north-western 
headland of Anglesey are marked on the Survey map as “ green, grey and 
purple slates with conglomeratic and siliceous beds.” The truly 'volcanic 
nature of a considerable proportion of these strata has been clearly stated 
by Mr. Blake.- As they dip in a general northerly direction, higher portions 
of the series present themselves as tar as the most northern projection of the 
island near Portli Wen (Fig. 58). They have been greatly crumpled and 
ciushed, so that the slates pass into phyllites. They include some thick 
seams of blue limestone and white quartzite, also courses of black shale con- 
taining Lower Silurian graptolites. Among their uppermost strata several 
(probably Bala) fossils, including Orthis Baihjana, have been obtained by 
Professor Hughes. It has been supposed that the higher bands of black 
shale may also have been brought into their present positions by faults, and 
that they do not really belong to the series of strata among which they lie. 
But this suggestion is completely disproved by the coast-sections, which 
exhibit many thin interstratified leaves of black shale, sometimes less than 
an inch thick. These and the ashy layers containing the OrtMs and other 
fossils form an integral part of the so-called “ Amlwch slates.” ® 
As evidence of the regular intercalation of the black shales and tuffs in 
this sedimentary series, a portion of the coast section at Porth Wen is here 
I cannot admit that there is any evidence of a thrust-plane here, 
modify iield-cvideuce to suit theory. See Oeol. Mag. 1891, p. 483. 
2 Quart. .lourn. Geol. Sac. vo!. .xliv. (1888), p. 517. See his further 
1891, p. 483. 
3 The Ami vvch slates e.xhibit on a great scale the puckering that points to intense compression. 
I his gnarled structure, as Prof. Hughes called it, has been illustrated by Mr. Harker, British 
Assoc. Iteport (1885), pp. 839, 840. 
To insert one is merely to 
remarks in Geol. Mag. 
