Geology and Mineralogy. 271 
of two bands of quartz-felsite (75°3 p. ¢. SiO,), six feet wide, 
enclosing in the center a band four feet in width of pitchstone 
porphyry (72-4 SiO,). Another dike, one of a series at Tormore, 
twelve feet in thickness and extending as a sheet for a long dis- 
tance, consists of the following bands in order between walls of 
sandstone: augite-andesite, banded and spherulitic felsite (ande- 
site), pitchstone, felsite (as before). Still another dike of the 
latter region shows a band of the pitchstone-porphyry cutting 
obliquely across both the bands of exterior augite-andesite and 
the quartz-felsite within it. 
After giving a detailed account of the structure and composi- 
tion, mineralogical and chemical, of these dikes, the author con- 
cludes with the following summary : 
“Any suggestion concerning the possible accidental association 
of the augite-andesite and ‘ pitchstone’ in the Cir Mhor dyke is at 
once negatived by the study of the remarkable plexus of dykes at 
Tormore. No one can doubt, after the study of this latter case, 
that there is a real and not merely an accidental connection be- 
tween the ejection of materials of such very different composition 
and character : all the facts, indeed, point to the conclusion that 
the fissures were injected from the same subterranean reservoir, 
but that this reservoir contained two magmas of totally different 
chemical composition. In the same way, as is well known, a sin- 
gle volcanic vent may give rise at successive periods to two totally 
distinct kinds of lava. 
“Nor can there be any difficulty in understanding how the same 
fissure, while still in connection with a reservoir of liquefied lava, 
may be reopened and re-injected at successive periods. The plane 
of weakness, along which the reopening of the dyke is effected, is 
sometimes, as in the Cir Mhor dyke, in its center; in other cases, 
as in the great north-and-south dyke of Tormore, and also in the 
most southerly of the transverse dykes of the same district, planes 
of weakness are found along one or both of the lateral walls of the 
dyke, and it is here that the re-injection is effected; in yet other 
cases, also illustrated at Tormore, the new fracture seems to be 
quite irregular in position and to traverse the old dyke-material 
in a sinuous line. In one instance we have evidence of three sep- 
arate injections into the same fissure. 
‘In some cases the more acid rock (quartz-felsite and pitchstone) 
was the first ejected; but, quite as frequently, the basic material 
(augite-andesite) was the earliest to be intruded into the opening 
fissure. The relative ages of the two rocks in the dyke are shown, 
not only by the positions which they occupy, but by the circum- 
stance that derived minerals from the older rock are found in- 
cluded in the younger one. That a very considerable interval of 
time must have elapsed between the two injections is shown by 
the fact that complete consolidation and crystallization of the ma- 
terials of the one rock must have occurred before its invasion by 
the other rock : this is proved by the characters of the junctions 
and also by the derived crystals. 
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