158 
The American Geologist. 
March, 1891 
the dyke. This change, even though it be due in great part to 
paramorphism, testifies to an important variation in the character 
of the rock developed from the same magma at various distances 
from the dyke walls. The augite nearer the middle of the dyke 
would appear to be much more susceptible of paramorphic change 
than that near the dyke walls. The middle part of the dyke is 
also richer in accessory minerals such as apatite, biotite, epidote, 
and leucoxene than the lateral parts. 
Chemical variation. — The chemical variations observed in this 
dyke will be gathered from a consideration of the following table 
of analyses of the specimens above referred to : 
Stop Island dyke. 
I. 
II. 
in. 
IV. 
Si0 2 
47.83 
47.08 
47.84 
57.50 
Fe 2 3 -fFeO 
4.57 
6.72 
5.07 
A1,0 3 
30.28 
25.40 
23.44 
CaO 
6.72 
8.44 
5.62 
MerO 
4.32 
5.25 
2.76 
K 2 
trace 
.60 
.45 
Xa o 
1.30 
2.55 
2.01 
P 2°5 
2.19 
.94 
2.02 
Loss on ig. 
2.05 
2.53 
" 25 
99.26 
3.060 
100.27 
101.12 
Sp. g. 
3.028 
3.080 
2.856 
These analyses show a remarkable increase in the propor- 
tion of silica in the middle of the dyke over that in the lateral 
parts. The difference in silica content of about 10 per cent is 
sufficient to separate the specimens into two distinct rock species 
according to current methods of classification. The difference is 
in keeping with the quartzose character of the middle of the dyke 
as compared with the quartzless character at the side, and also 
harmonizes with the difference in specific gravities given in the 
table. 
Thus in half the space of a sharply defined dyke only 150 
feet wide our .study reveals variations in all of those characters 
which we make use of in the description and classification of rocks. 
Totally distinct types, of texture, structure and composition belong- 
to the same geological unit-mass. This fact suggests an interest- 
ing commentary upon our system of rock classification. Is such 
classification in cases like the present, or even generall}*, anything 
