DOLEPJTES. 
137 
In certain specimens (698 from Dry Valleys and 661 from Knob Head) the 
micropegmatitic patches are not so prominent, but quartz is still present in the 
ground-mass. These rocks are of somewhat finer grain, and consist of a base, of small 
stout felspar-prisms with a little quartz, from which the colourless augites stand out 
conspicuously in large irregular ophitic plates. 
The augite in these dolerites gives extinctions of over 40° ; the sections are for the 
most part without definite crystalline outline ; most of them show the usual twinning 
on 100, and many of them also exhibit the “ herring-bone ” structure due to fine 
striations parallel to 001 in the two halves of a twin.* 
The felspar-prisms in these rocks generally show Carlsbad-twinning. Albite- 
twinning is not so common, but in some sections showing twin lamellae symmetrical 
extinctions of 26° were observed. 
The result of a chemical analysis of specimen 661 from Knob Head is as follows 
under I. Under II is given, for comparison, the analysis of the “ augite-diorite ” from 
Madras referred to above. 
I. 
Ia. 
II. 
(Knob Head). 
Mol. ratios. 
(Madras). 
SiO., 
= 53-26 
•882 
51-15 
Ti0 2 
= 0-70 
•009 
0-44 
alo 3 
= 15-64 
•153 
15-92 
FeA 
= 0-24 
•001 
9-34 
FeO 
II 
•105 
2-87 
MnO 
= 0-11 
— 
0-09 
CaO 
= 12-08 
•215 
10-40 
MgO 
= 8-64 
•214 
6-48 
Na./> 
= 1-25 
•020 
1-19 
K„0 
= 0-58 
•006 
1-61 
PA 
= 0-04 
0 • 06 
H.,0 at 110° 
= 0-35 
- | 
O'll 
H 2 0 above 110° 
= 0-41 
- I 
100-74 
99-66 
The Osann formula for this rock is : — 
s 55'46 C'iifui 'h e;! 
which is near the formula for a basalt from Etna belonging to Osann’s “ Royat ” type, 
viz. : — 
S 54'21 ^sfu ^8 
Calculation of the “ norm ” gave the following result : — 
KAlSiA 
== 
3-34 
NaAlSiA 
= 
10-48 
SA 
= 
3-36 
CaALSiA 
= 
38-09 
CaSi0 3 
= 
9-05 
FeSi0 3 
= 
13-72 
MgSA 
= 
21-40 
FeO. TA 
= 
1-37 
FeO. FeA 
= 
0-35 
VOL. I. 
Schaub, Neues Jahrb., 1905 (i), p. 100, and Plate VI, Fig. 5. 
T 
