154 
THE CAMBRIAN VOLCANOES 
BOOK III 
gredient is olivine. Eecl liaematitic crystcals are visible, even to the naked 
eye, dispersed through the groundmass of the rocks. With a lens these 
may be observed to be orthorhombic in form, and to be evidently pseudo- 
morphs after some mineral wliich has been converted chiefly into haematite. 
Such red pseudomorphs are common in Carboniferous and Old Eed Sand- 
stone lavas, -where in some cases they appear to be after hornblende, and in 
others after augite, but occasionally are suggestive of olivine, thoirgh with 
no trace of the original sul.istance of that mineral. In the lava associated 
with the tuffs at the soirth end of the promontory between Eamsey Sound 
and Een-y-foel, however, certain large, well-developed pseudomorphs are 
undoubtedly after olivine. They have the characteristic contour of that 
mineral and its pieculiar transverse curved and irregular fractures. The 
average length of these pseudomorphs was found, from the measurement of 
six examples, to be 0'023 inch, the largest being 0'034, and tlie smallest 
0'014. Seen liy transmitted light they present a structureless jiale-green 
material nearly inert in polarized light, round the borders and across 
fissures in which an opaque substance has been developed, as serpentine 
and magnetite have been grouped in the familiar alteration of olivine. The 
opaque material is bright brick-red in reflected light, and is evidently 
now chiefly oxidized into haematite. Every stage may be traced, from 
orthorhombic forms with the incipient development of transverse fissures 
filled with iron-oxide, to others of distorted shapes in which the ferrugi- 
nous matter occupies the whole, or nearly the whole, of the mould of the 
original crystal. 
The rocks now described differ from the Palccozoic andesites or “ por- 
phyrites,” with which I am acquainted, in their more basic composition, in the 
less abundance of their microscopic liase, in the comparatively inconspicuous 
development of their felspars of later consolidation, in tlie absence of large 
porphyritic felspars of earEer consolidation, in the extraordinary prominence 
of the granular augite, and in the presence of olivine. In composition and 
structure they ai’e essentially forms of olivine-diabase. 
Above the volcanic group of St. David’s lies a band of quartz-con- 
glomerate which has been taken by Dr. Hicks 
to mark the base of the Cambrian system. This 
rock, though mainly composed of quartz and 
quartzite, contains fragments of the underlying 
volcanic rocks. But that it does not mark any 
decided break in the sedimentation, much less 
the violent uneonformability and vast interval 
of time which it has been erroneously supposed 
to do, is well illustrated by the occurrence of 
bands of tuff, as well as diffused volcanic dust, 
in the conglomerate and also in the green and 
red sliales and sandstones which conformably 
overlie it. An example of this intercalation of 
volcanic material is represented in Pig. 41. On the left side vertical layers 
Ip- 
« bed 
G. 41. — Section showing the inter- 
stratificatioii of tuff anil con- 
“ 1.®/.* 'I f\ 
