Geological Society of London. 91 
that they have been much cracked, and sometimes even partially 
kaolinized before growth recommenced in them. In some cases 
the crystals have been actually broken and recemented by newly 
deposited felspar-material. 
While there is a general crystallographic continuity between the 
old felspar-crystals and the new outgrowths from them, the varia- 
tions in the position of extinction in different portions of the 
enlarged crystal show that, as growth went on, the composition of 
successively formed zones gradually and progressively changed from 
near the anorthite limit to close upon the albite limit. 
These facts prove that, under suitable conditions, felspar-crystals 
in solid rock-masses may grow at the expense of the unstable glass- 
magma by which they are surrounded. This conclusion is in com- 
‘plete harmony with some other recent researches—especially those 
of Dr. J. Lehmann on the mode of production of the perthite- 
structure in felspars. In conclusion, the circumstances which have 
given rise to the exceptionally clear illustration of the processes 
described in the rock under consideration were explained, and the 
bearings of the principles enunciated on the theory of metamorphism 
are indicated. 
2. “The Tertiary Volcanoes of the Western Isles of Scotland.” 
By Prof. J. W. Judd, F.R.S., ¥.G.S. 
In his recently published memoir, ‘‘ The History of Volcanic 
Action during the Tertiary Period in the British Isles,” Dr. A. 
Geikie, while adopting many of the views propounded in a 
communication made to this Society in 1874, “On the Ancient 
Volcanoes of the Highlands,” takes exception to certain of the 
conclusions which are maintained in that paper. 
Among the ideas set forth in 1874, of which Dr. Geikie now 
announces his acceptance, and to which, indeed, he supplies valuable 
support and confirmation, from his own observations and those of 
various members of the Geological Survey, are the following :— 
(1) The perfect transition between the plutonic rocks of the 
district (granites and gabbros) and the lavas (‘‘felstones” and 
basalts), and the dependenee of each variety of texture exhibited by 
them—from the holocrystalline to the vitreous—on the conditions 
under which solidification took place. 
(2) The presence of great masses composed of volcanic agglomer- 
ates, breccias and tuffs, with numerous intrusive bosses, sheets, and 
dykes, at five well-marked eruptive centres, namely Mull, Ardan- 
murchan, Rum, Skye, and St. Kilda, and the subaerial character of 
the ejections at these five centres. 
(3) The Tertiary age, not only of the lavas, but also of the 
gabbros and granites found associated with them at these different 
centres. 
The conclusions to which exception is taken are as follows :— 
(1) That the ejection of the “felstone”’ lavas and the intrusion 
of the granites preceded the appearance of the basalts and gabbros. 
(2) That the five centres of eruption mark the sites of as many 
great volcanic cones, now ruined and dissected by denudation. — 
