46 
president’s address. 
but with axial proportions which make its planes approximate 
very closely to those derived from a cube. Recent observers, 
however, have found that by cautiously heating a slice of a 
ieucite crystal there arrives a time when the double refrac¬ 
tion, and therefore of course the twinning lamellae, disappear, 
and the plate becomes singly refractive as a cubic crystal. 
At its time of formation, therefore, Ieucite is probably a 
cubic crystal, but cooling induces other conditions, and the 
crystal changes to accommodate itself to them. Orthoclase 
is another mineral which undergoes great changes on heating, 
though in this case the results as observed by ordinary means 
are not so immediately striking. 
Closely connected with this most interesting subject is 
the grand work of Dr. Lehmann on the origin of the 
crystalline schists, with special relation to the Saxon granu- 
lite region. The schistose rocks of this district have of late 
been classed as Archaean, but the author of the work 
considers that they are metamorphosed Palaeozoic sediments 
of which the metamorpliism probably took place between 
the Devonian and Carboniferous periods, when the district 
was crumpled and uplieaved. Among them, however, he 
traces various masses of gabbro, evidently eruptive, through 
their various changes of substance and arrangement, until 
they result in hornblende schists in just the same way as 
the Scotch dyke which I have already mentioned. Another 
important observation of Lehmann’s is the increased quantity 
of biotite, as the rocks are followed inwards from the less 
altered slates and shales from which he considers them to 
have been derived. The magnificent atlas of plates which 
illustrate the work is not its least important part. The photo¬ 
graphs, to the number of more than 150, represent typical 
hand specimens, and are, in many cases, either reduced in 
size, or of about the natural dimensions. Those which show 
the effect of pressure upon solid rocks, grinding them and 
making them flow, as if liquid, around the more intractable 
pieces, are strikingly like some examples which Professor Lap- 
worth has on a microscopic scale from northern Scotland. 
The deforming of the felspar crystals which yet have survived 
the crushing is exactly similar to that which I have had the 
honour of exhibiting to the society in this room. 
The meeting of the British Association in Montreal last 
autumn was one of those experiments of which the success 
is the great justification. Of the success there seems to be 
no doubt whatever, in spite of all the dismal prognostications 
which greeted the decision of the committee. The special 
facilities for travelling and the generous hospitality of the 
