CHAP. X 
S7'. DAVID’S 
157 
formations. The dykes are specially abundant in the granite. One or two 
may be detected in almost every artificial opening which has been made in 
that rock ; while on the coast-section they are here and there profusely 
abundant. Tliey are likewise frequent in the quartz-porphyries, and occur 
also in the volcanic group as well as in the sandstones and shales above the 
conglomerate, but become fewer in number as they recede from the granite 
centre.^ 
In external characters, the rock composing these dykes and sheets may 
be described as usually a dull dirty-green or yellowish-brown mass, to 
which the old name of “ wacke ” might appropriately be given. It exhibits 
the texture and mode of weathering of the moi’e distinctly crystalline 
members of the basalt family. It is occasionally amygdaloidal or cellular, 
the kernels or cavities being arranged parallel with the sides of the 
dyke. Here and there a rudely prismatic structure extends between the 
walls. 
The microscopic structure of this rock has been described by Professor 
Judd, Mr. Davies and Mr. Tawney. It is a diabase, but more allied in 
structure to true basalt than the olivine-diabase of the volcanic group. It 
especially differs from the older rock in the abundance and freshness of its 
felspars, in the comparative scarcity of its augite, and in the absence of 
olivine. The magnesian silicates are very generally replaced by green 
decomposition-products diffused through the mass. An occasional crystal 
of hornblende, recognizable by its cleavages and dichroism, may be detected. 
Some of the diabase dykes present excellent examples of How -structure. 
A beautiful instance occurs in a dyke that cuts the shales, in a small 
oove to the east of Nun’s Chapel. The shale and eruptive rock are in 
contact; and the small acicular prisms of felspar, besides ranging them- 
selves in line parallel to the side of the dyke, stream round the larger 
felspar crystals. 
Some of the dykes or veins are only three inches broad. They send 
out fingers, and sometimes break abruptly across from one line to another. 
They appear generally to have hdlowed the lines of joint in the granite, as 
Mr. Tawney has observed ; ^ consequently they must be posterior to the 
development of the system of jointing in that rock. 
Besides the abundant dykes, there has been a more limited extrusion of 
the same material in sheets parallel (or approximately so) to the bedding of 
the sandstones and shales. These sheets are well displayed at &t. John s 
Point, where evidence of their being intrusive, and not truly bedded, may 
be seen alonw the fine cliff's which have been cut by the waves on this part 
o 
of the coast-line. 
The sedimentary series which overlies the volcanic group of &t. David s, 
and contains the fossils of the lower part of the Cambrian system, gradually 
loses all trace of volcanic material, as its members are followed upward in 
^ The occurrence of these dykes is iiaralleled by that of the similar intrusions in the quartz- 
felsite of Llyn Padarn to be afterwards described. 
“ Proc, Nat. Sisl. Hoc. Jiristul, voh ii. part ii. (1879), p. 115. 
