934 J.J. H. TEALL—PETROLOGICAL NOTES ON 
Newcastle, occurs the well-known Coley-hill dyke, which was for- 
merly worked on a very extensive scale for road-metal, and the 
course of which is now indicated by adeep trench. This dyke agrees 
very well in direction with the Tynemouth dyke, and Winch appears 
to have regarded the two as connected. 
I was not able to find the Coley-hill rock in situ; but some large 
angular fragments were found in the trench at one point, and as 
these agree with the description gived by Winch, I have no doubt 
that they were portions of the dyke. If so, they prove that the 
petrological characters of the Coley-hill dyke agree with those of 
the Tynemouth dyke, and differ from those of the Hebburn dyke, a 
fact which seems to establish the connexion of the two former. 
I now proceed to describe the rock of the Tynemouth dyke. It 
varies in aspect according to the presence or absence of (a) porphy- 
ritic crystals of anorthite and (6) small spherical amygdaloids. 
The porphyritic crystals frequently measure more than 1 centim. in 
cross section ; they are, as a rule, very irregular, and consist rather 
of crystalline aggregates than of simple crystals; nevertheless 
sections parallel to the basal plane, showing the forms « Po, « P’, 
and oo 'P, arenot uncommon. These sections show also the striation 
characteristic of plagioclase. ‘The crystals are glassy in texture, but 
they frequently show a slight tinge of yellow. Isolated fragments 
of these felspars were analyzed by my friend Mr. Stead, with the 
following result :— 
DileAae Ly ep eee eee 47°30 
Alimnimaeerree Le ee 31°50 
WELECHOXIGe lacey ae 1°85 
Ibrmeye ahs} aol ere 14°88 
Mapnesiats: cet See ee 0°93 
IROfaShIOe: A Rie Ce 38 
SXOY | RRM GAR Se oe BE 1-22 
THOSE Ste Coa aoe eee 1°80 
99°86 
It is impossible to obtain the felspar in a state of absolute 
purity, on account of the numerous inclusions which it contains; and 
it will also be shown further on that the outer zone of felspar 
substance possesses optical properties somewhat different from the 
central portion ; nevertheless the analysis clearly proves that the 
felspar is closely allied to anorthite. A section of the felspar 
specially prepared and approximately parallel to the basal plane 
gave 55° 10’ as the angle between the extinction-positions of 
adjacent lamelle. The corresponding angle in anorthite, according 
to Lévy and Fouqué, lies between 57° and 74°, and in labradorite 
between 10° and 14° 30’. Assuming Schuster’s interpretation of 
Tschermak’s theory to be correct, then this figure corresponds to 
about 90 per cent. of anorthite in the mixture. The above facts 
with regard to the Tynemouth felspars have appeared in the Geol. 
