SOME NORTH-OF-ENGLAND DYKES. 243 
points near Newton and Clennel, 17 or 18 miles further west. Also 
on the hills about 2 miles north-west of Alwinton, at the point where 
it crosses the Usway, half a mile above Shillmoor Farm, and, lastly, in 
the road along the Coquet, one mile above Shillmoor. At the two 
last-mentioned exposures the dyke is intrusive in Cheviot porphy- 
rites. 
Observations based on numerous specimens collected from the 
above localities prove that the rock of this dyke, like that of the 
Cleveland, Cockfield, and Armathwaite dyke, is remarkably constant, 
both in its macroscopic and microscopic characters, throughout the 
entire length examined. It is distinctly crystalline in texture, and 
of a bluish-grey colour when freshly broken. The colour, however, 
rapidly changes to black on exposure. Porphyritic crystals of fel- 
spar are not to be recognized by macroscopic examination ; and in 
this respéct there is a marked difference between the Acklington 
and Cleveland dykes. Determinations of the specific gravity of 
specimens from each of the above localities were made, with results 
varying from 2°805 to 2°817. The average may be taken as 2°81 ; 
and the deviations from this average, therefore, do not appear to 
reach more than two fifths per cent. Mr. Stead has analyzed the 
rock with the following result :— 
SHUG Ei tis aw wl eee 07°30 
ARTISTTNT eas ea ee 16°18 
MerricioxG@e a ani). 10:07 
1ST 1 ae ek eee 6°18 
INMnomeslarcnret tacit is. nece.< 4-68 
iPOGAgIN 4 Fee ee 0-77 
SOG) tae A Ree a 2°38 
Web CBee iret ove, joo wre ores 1-70 
99°76 
Under the microscope the original constituents are seen to com- 
prise felspar, pyroxene, magnetite, and interstitial matter, with 
various devitrification-products. The general structure reminds one 
of the ground-mass of the Cleveland, Cockfield, and Armathwaite 
dyke, except that the interstitial matter in this case is much less 
abundant. 
Felspar.—The prevailing felspar occurs in lath-shaped sections 
similar in every respect to the small felspars of the Cleveland dyke. 
A detailed description is therefore unnecessary. In the majority of 
slides examined these are the only felspars to be recognized ; but in 
one or two other forms occur which belong to an earlier generation, 
and therefore give the rocks a porphyritic character. These give 
sections more equal in the different directions ; they contain nume- 
rous inclusions, and are usually composed of a central nucleus and 
a peripheral zone, which extinguish in slightly different positions. 
Pyrowene occurs in crystalline grains and small imperfectly deve-. 
loped crystals, the latter sometimes elongated in the direction of the 
* Total iron reckoned as Fe,0,. 
B2 
