SOME NORTH-OF-ENGLAND DYKES. 245 
crystalline and granular; whereas the others contain a considerable 
amount of indistinctly individualized matter. All the dykes show 
occasional traces of two generations of felspar; but the only ones 
which are pooepcu ously porphyritic are those of Cleveland and 
Tynemouth. 
The observations which have been made on the Cleveland, Ack- 
lington, and Hett dykes prove that the minute characters, both of 
composition and structure, are constant throughout great distances, 
and bear no relation to the surrounding rocks. These observations 
do not lend any support to the view that the igneous matter has taken 
up any appreciable portion of the sedimentary material into which it 
has penetrated. If we endeavour to give names to the rocks here 
described we are met by the difficulty arising from the absence 
of unanimity among petrologists as to the use of such terms as 
basalt, dolerite, augite-andesite, augite-porphyrite, diabase, and 
melaphyre. 
In this country we find it quite impossible to apply any system of 
classification which primarily depends on a distinction between 
pre-Tertiary and post-Tertiary rocks, for the following reasons :— 
(1) The age of many of our eruptive rocks, including : some cf the 
dykes here referred to, cannot be determined by field-evidence ; (2) 
rocks identical both in structure and composition, and belonging to 
the basic, intermediate, and acidic groups, have been formed at 
widely separated intervals of geological time. 
If we assume, for the purpose of indicating the relations of our 
rocks with these of thec ontinent, that groups 2, 3, and 4 are of 
pre-Tertiary age, then the Hett and High-Green dykes would have 
affinities with the diabases and augite-porphyrites, but would be 
more allied to the former than the latter. The High-Green dyke 
might almost be called a typical diabase ; for the interstitial matter 
is hardly present in recognizable quantity, and the porphyritic fel- 
spars are very few and far between; they cannot be recognized 
without the microscope. In the mode of alteration of the rock and 
the character of the ilmenite it exactly resembles many of the 
continental diabases. The dyke to the north of High Green is 
more distinctly porphyritic, and indicates, therefore, a transition 
from the diabases to the melaphyres. 
The dykes of, group 3 would, on the continent, be described as 
melaphyres ; indeed Prof. Rosenbusch speaks of a rock from New- 
castle-on-[yne, probably a portion of the Tynemouth and Coley- 
_ hill dyke, as a true melaphyre (‘ Massige Gesteine,’ p. 410). 
It is worthy of note that many of the rocks to which the term 
“‘melaphyre” has been applied have a higher silica percentage 
(about 56 per cent.) than dolerites and basalts. Thus they are 
described in the older petrology as consisting essentially of oligo- 
clase and augite, whereas the felspar in-the basalts is described as 
labradorite (Zirkel, Lehrb. d. Petrogr. 1866, Band i1. p. 39). If this 
view were generally accepted, then the melaphyres would bear the 
same relation to the augite-andesites as the diabases do to the 
dolerites. 
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