The Wackerfield Dyke. 451 
saturated from oversaturated material became possible. Within 
each type of magma the crystallization and slight upward con- 
centration of plagioclase (lighter than the magma), accompanied and 
followed by a slight downward concentration of the heavier 
black ores, would then take place. Such a process, being continuous 
in depth, but arrested locally by intrusion or eruption, would appear 
to explain the antipathy between the constituents mentioned. It is 
suggested that the Wackerfield dyke, the Hett dyke, and the Whin 
Sill are related by some such community of process, and that they 
were drawn either from a single uniform magma or from a series of 
uniform magmas. 
Moreover, chemical, mineral, and textural evidence shows not 
only that the parent magma was similar to that from which the 
oversaturated Brito-Arctic basalts were derived, but that the 
processes which acted on each were in all essentials identical. This 
conclusion does not warrant the deduction that the Whin Sill and the 
related dykes are of Tertiary age, for that would imply that similar 
processes acting on similar materials could take place only during 
a single period of the earth’s history. On the contrary, many features 
of late Carboniferous magmatic history are known to have been 
repeated in the igneous episodes of the Tertiary, and correlation of 
process between certain Tertiary rocks and the Whin Sill group 
must therefore not be confused with correlation in time. 
Chemical Composition—We are indebted to Dr. H. F. Harwood 
for the following excellent analysis of specimen No. 6 :— 
Molecular 
Constituents. Percentages. Proportions. Norm. 
Sid... i : 50°30 838 Quartz . : 5:22 
Al,Os 3 14-11 138 
Fe,03 . 3 : 3°16 20 Orthoclase sf 4°38 
FeO : : j 9:90 138 Albite . i 19:78 
MgO . : : 5:56 139 Anorthite ; 25°58 
CaO ’ 8°54 153 
Na,O 2°34 38 Diopside ‘ 11-40 
KG OR. 6 0-74 8 Hypersthene . 20°40 
H,0 (> 105° C.) 1-94 sa, 
HO (at 105° C.) 1-01 — Magnetite , 4:55 
CO, z , 0:16 4. Ilmenite . : 4:22 
TiO, 222, 28 Apatite . . 0°61 
P.O; 0:30 2 Calcite . , 0:36 
MnO. A 3 0:12 2 —— 
BaOie. : 4 0:03 — 97°50 
SrOws : . none found — Water . : 2:95 
Si. : ; . none found — 
Cl ! ; : trace — Total . 100°45 
ZrO, . y A trace — 
—-— Specific) __ 5, 
Hotaliene. : 100-43 ee, = atk 
The analysis is recalculated into standard mineral compounds 
(normative minerals), and restated in this form on the right. The 
mineralogical characters of the rock are faithfully reflected by the 
standard mineral composition ; that is to say, the mode is nearly 
