J. J. Harris Teall—Oheviot Andesites and Porphyrites. 259 
Such an explanation is certainly true of similar cases in the rock 
forming the Cleveland Dyke, in which the outer zone of felspar 
substance in the porphyritic crystals frequently contains minerals 
of the ground-mass, and extinguishes at different angles from the 
central portions. 
I submitted a fragment of one of these large crystals to Mr. 
Waller, of Birmingham, for examination by Szabo’s methods, and 
he reports that both the flame reactions and fusibility, or rather 
infusibility, show that it is practically identical with the porphyritic 
crystals of the Tynemouth dyke, portions of which isolated by 
myself have been kindly analysed for me by my friend Mr. J. E. 
Stead, of Middlesborough, with the following result :— 
Si02 aan ce earn 47°30 
OA Ome erate ek ak saat fat, Oc ar ae omnes ea ma ene O)) 
1 BS Os ef Mal sO as ls pele as AR aR ek AR 8 1°85 
CaO Bris testes Ee ue ieee ort 14°88 
MgO - 0°93 
K,0O 38 
Na,O 1:22 
Loss 1:80 
99-86 
It is impossible to obtain the felspars in a state of absolute purity, 
on account of the numerous inclusions which they contain, and also 
on account of slight alteration; nevertheless the analysis clearly 
proves that we are dealing either with anorthite, or the more doubt- 
ful species bytownite. With considerable difficulty I have prepared 
a section of this felspar approximately parallel with the basal plane. 
In this section the angle between the extinction positions of adjacent 
lamelle is 55° 10’: 
The corresponding angle in anorthite, according to Levy and 
Fouqué,! lies between 57° and 74°, and in labradorite between 10° 
and 14° 30’, Assuming Schuster’s interpretation? of T'schermak’s 
theory to be correct, then these figures correspond to about 90 per 
cent. of anorthite in the mixture. 
It thus appears, both from the chemical and optical examination, 
that the Tynemouth felspar is either bytownite or anorthite. 
A few crystals which had weathered out were submitted to Dr. 
Trechmann of Hartlepool for crystallographic examination. He 
writes, “I have now measured two of your crystals successfully. 
Though unpromising in appearance, yet one gave good reflections 
and the other fair ones. The angle measured was the inclination of 
the basal planes to one another in the twins. The brachypinakoid 
was not suitable, it is inclined to irregularities, i.e. the two cor- 
responding faces of a P& are not strictly parallel, a frequent occur- 
rence in nature. I found 
Crystal No. 1. 
Cte OO = Nil 98/ irom, which 
oP:aP€ (acute angle) = 85° 59’ 
1 Min. Mic. p. 230. 
* Ueber die optische Orientirung der Plagioclase, T. M. M., 1881, p. 254. 
