244 J. J. H. TEALL—PETROLOGICAL NOTES ON 
vertical axis. Twinning is common, and cleavages are sometimes 
well shown. In thin sections this mineral is almost colourless. 
Magnetite is present as small crystals, grains, and skeleton- 
crystals. 
The interstitial matter is similar in every respect to that which 
is so common in the Cleveland dyke, and does not therefore call for 
special description. The secondary products are calcite and quartz. 
Pyrites also occurs. Amygdaloidal cavities containing a crystal of 
calcite surrounded by a narrow zone of chloritic material are found 
both at Debdon and Clennel. 
The dyke is not known to intersect any formation of later date 
than the Coal-measures; but from the fact that it bears a close re- 
semblance in character and direction to that of — I am 
disposed to regard it as of Tertiary age. 
ConcLuUsIoN. 
If we reconsider the facts recorded above, it will appear that the 
dykes to which reference has been made fall into four more or less 
distinct groups :— 
1. (a) Cockfield, Cleveland, and Armathwaite dyke. 
(6) Acklington dyke. 
2. The Hett and related dykes. including the “sill” in the 
Browny and Page-Bank collieries. 
3. (a) The Hebburn dyke. 
(6) The Tynemouth dyke. 
(c) The Brunton dyke. 
(d) The Seaton and Hartley dykes. 
(e) The Morpeth dyke. 
4, The High-Green dykes. 
Groups 1 and 3 resemble each other in chemical composition 
and specific gravity; so also do groups 2 and 4. The former 
have a silica percentage varying from 57 to 59, if we except the 
somewhat abnormal Morpeth dyke (which appears in other respects 
to belong to group 3), and a specific gravity of about 2-7 or 2°8; the 
latter have a silica percentage varying from 51 to 53, and a some- 
what higher specific gravity. Groups 1 and 3 are distinguished 
from each other by certain peculiarities of microscopic structure, 
which may or may not be related to a difference in geological age. 
Groups 2 and 4 are closely related both in composition and 
structure, the principal difference being a pathological one. Altera- 
tion-products abound in group 4, but are for the most part absent 
from group 2. 
If we consider the dykes with reference to the structures indi- 
cated by the terms “ porphyritic,” “ granular,” and “ holocrystal- 
line,” as these are used by Prof. Rosenbusch, we find that the Hett 
and the High-Green dykes are almost, though not entirely, holo- 
