The Wackerfield Dyke. 44.9 
generally 1:uch smaller and of wedge-like shape, are occupied by a 
fine granular aggregate, which appears to be mainly quartz. How- 
ever, it is not wholly quartz, as the material has a pale-green colour, 
and examination under high power reveals the presence of a fibrous 
green chloritic mineral and of occasional minute flakes of sericite 
and rare blebs of a carbonate mineral. In other parts of the rock 
irregular spaces are occupied mainly by cloudy aggregates of calcite. 
No true amygdales occur, but it seems likely that the infilling of the 
final residual spaces represents a late magmatic stage of con- 
solidation corresponding to that which fills the amygdales of the 
Whin Sill and the Hett dyke. The effects of this stage of the cooling 
history are seen most conspicuously in the altered peripheral portions 
of the intrusion. 
Other Minerals ——Careful search through a series of thin sections 
revealed no additional minerals except a clear needle-like crystal of 
apatite associated with the calcite aggregates. No mica or pyrite 
has been detected in the fresh rock. 
Mineral Analysis—A series. of micrometric analyses by the 
Delesse-Rosiwal method gave the following percentages by volume. 
Results for a specimen of the Hett dyke and for two specimens of 
the Whin Sill are added for comparison. 
Wackerfield Dyke. Whin | Whin 
Minerals. F Heit | Sill.2 | Ss 
: No. 1.| No. 6. | No. 11. | Dyke. 
Felspars . 556 54:0 | 52-7 52:3 53:7 48:9 
Pyroxenes 34:5 30:2 31:8 30°8 33:1 39°8 
Black Ores 5:2 6-6 7:2 6:8 5°8 7:2 
Interstitial 4:7 9:2 8:3) 4) LO 7:4 4°] 
Calelated . 2-96 2°98 3'01 2:98 2°98 3°04 
Spec. 
Grav.) Determined 2°96 2°96 PNG) Ik a8 5) 2,94 3°01 
The calculated percentages by weight are set forth below, together 
with the specific gravities (approximate only), used for the groups of 
minerals measured. 
Wackerfield 
iW Spec. Average. Hett | Whin | Whin 
Linerals. Grav. Dhkye. | Sill. | Sill. 
Volume.| Weizht. 
Felspars . Dall 54:1 49-0 47-4 48-7 43°4 
Pyroxenes 2 322 34:5 33:0 38°6 41:8 
Black Ores 4:8 6°3 10°1 10°8 9°3 11:3 
Interstitial 2:6 74 6-4 8:8 6:4 3°5 
1 Lunedale, Yorkshire. 
2 Kirkwhelpineton, Northumberland. 
3 Greenhead, Northumberland. 
VOL. LVIII.—NO. X. 29 
