van 
\ 
The Wackerfield Dyke. © 453 
incapable of providing a satisfactory basis for chemical comparison. 
The two analyses of Tertiary rocks from Mull are given to demon- 
strate the similarity to which reference has already been made. 
The Cleveland dyke, like the Whin Sill, is extraordinarily uniform 
throughout its range, and the uniformity i is borne out by a number 
of fairly good analyses. One of these is quoted in the table above, 
in order to bring out the fact that it is of andesitic rather than 
doleritic composition. The Wackerfield dyke evidently belongs to 
a different suite from that of which the Cleveland dyke is the type. 
It is possible that between the tholeiite group of the Northumberland 
dykes, the Whin Sill group, and the Cleveland dyke group there are 
genetic relations similar to those connecting the  tholeiites, 
cumbraites, leidleites, and inninmorites of the Clyde area.t Here, 
again, differences of geological age have no necessary bearing on 
the question, but in the absence of sufficient adequate chemical 
data, discussion of this petrological problem must be deferred. 
ALTERATIONS. 
As already stated, the dyke-rock does not depart from a uniform 
type except where the minerals have suffered a marked alteration. 
This alteration is largely but not entirely marginal, and there the 
rock is slightly—-but very slightly—finer in texture. 
Microscopic examination indicates that the alterations are of two 
distinct kinds: one related to silicification and the other to 
carbonatization. 
Specimen No. 7 (taken from floor of quarry and from near the 
centre of the dyke at a depth of about 20 feet or more—Exposure 1) 
is typical of the latter. The rock is pale grey with occasional 
splashes of brown limonitic staining. In thin section it is seen that 
the pyroxenes are replaced by “cloudy granular aggregates of 
carbonate minerals, penetrated in places. by numerous acicular 
crystals of apatite, and associated with sporadic grains of pyrite. 
Certain vaguely defined rhombs suggest the presence of dolomite 
in addition to calcite, and this is confirmed by chemical tests. The 
felspars are slightly fooged, but twinning and zoning still remain. 
Small flakes of biotite have developed, especially between titaniferous 
magnetite and the interstitial matter, suggesting an interaction 
between these minerals promoted by the percolating solutions 
either during or after their attack on the pyroxenes. The inter- 
stitial material as a whole remains unaffected. 
Specimen No. 4 (from the north cheek of the dyke immediately 
east of the sill—KHxposure I) is a brownish-green rock with a faintly 
purple coloration here and there. The dominant alteration is the 
replacement of the felspars by quartz, chalcedony, and opal. The 
opal appears in core-like granules and streaks, these, surrounded by 
chalcedony, give the felspars a mottled relief due to rapid alternations 
1G. W. Tyrrell, Gror. Mac., 1917, pp. 305 and 350. See also 8. Tsuboi, 
Journ. Geol. Soc. Toyo, xxv, 1918, p. 47. 
