SOME NORTH-OF-ENGLAND DYKES. 933 
and ragged terminations, and green pseudomorphs after a pyroxene 
having definite crystalline boundaries ; both the six- and eight-sided 
forms characteristic of sections of augite parallel to the orthopina- 
koid, and at right angles to the vertical axis, may be easily 
recognized. The ground-mass is a confused aggregate of extremely 
minute felspar microlites, specks of opacite, and indistinct brownish 
granules. Isotropic glass may or may not be present ; a considerable 
amount of calcite in the form of crystalline powder is scattered 
throughout the mass. 
The absence of well-formed augites in the main mass of the dyke, 
and their presence in the more rapidly cooled marginal portions, is 
a fact of some considerable importance, as apparently showing that 
the period of consolidation of the augite is dependent rather upon 
physical than upon chemical conditions. In microscopic structure 
the rock of the Hebburn dyke differs from that of either of the dykes 
already referred to and approximates very closely to that of several 
dykes now to be described. 
Is this dyke of Tertiary or pre-Tertiary age? No definite answer 
can be given to this question. Itis not known to cut any formation 
of later date than the Coal-measures. 
Tue Tynemoutu Dyxkr. 
LIaterature. 
Winen, N. J. “Geology of Northumberland and Durham,” Trans. 
Geol. Soc. vol. iv. 
Lezour, Prof. G. A. Outline of the Geology of Northumberland 
(1878). 
Ordnance Maps 105 N.E. and N.W. 
This dyke is exposed at the base of the Castle Rock, Tynemouth. 
It may be examined on the shore in the angle formed by the meeting 
of the breakwater and the coast-line, where it is seen in contact 
with Coal-measure strata on the north, and with the wall of the 
breakwater on the south. Permian strata cap the hill on which the 
Priory stands; but the dyke is not seen in contact with these beds. 
It is about 10 feet wide, and separated into two portions by a 
quartzose vein which is 6 inches wide. The same dyke was also 
admirably exposed during the construction of the new railway 
station at Tynemonth in the winter of 1881 and 1882. 
The most interesting feature connected with this exposure was 
the evidence of a breach in the continuity of the dyke, accompanied 
by a lateral shift in the outcrop, amounting to 17 yards. The width 
of the dyke was about 11 or 12 feet, and it possessed a hade to the 
north. Further west the same dyke has been met with near Biddy 
Mill * by Mr. Flavell, F.G.S., during the construction of works by 
the North Shields Water Company. Still further to the west, near 
* Lebour, ‘Geology of Northumberland,’ p. 48. 
