230 NATURAL SCIENCE. April. 



As regards the first point, Dr. Becker gives reason for believing 

 that no appreciable effects could be produced by diffusion during any 

 time which can reasonably be postulated. This would be true even 

 in aqueous solutions of salts ; in natural rock-magmas viscosity would 

 further check diffusion, while convection-currents and disturbance by 

 mechanical means would speedily undo its work. He considers that 

 viscosity and other causes would also prevent the separation of 

 immiscible fluids into distinct bodies occupying different spaces. As 

 no considerable superheating will be possible in subterranean magmas 

 in contact with solid rocks, the range of temperature must further be 

 very limited. 



Without attempting to meet the author's arguments, we may 

 remark that the two hypothetical processes which he examines do not, 

 as he seems to assume, exhaust the possibilities. For instance, he 

 passes over the effects which may be expected to accompany 

 crystallization in a slowly cooling body of rock magma. It can 

 scarcely be doubted that such crystallization is accompanied by a very 

 considerable evolution of heat. Hence we should expect th^t, when 

 crystallization has begun in the coolest portion of the magma, the 

 tendency to degradation of energy will determine a movement of the 

 constituent about to crystallize to that place, so as to maintain 

 saturation there. 



Nor must we lose sight of the possibility that mechanical forces 

 operating during the progress of crystallization may be an important 

 factor in differentiation. Dr. Becker ascribes the diversity of igneous 

 rocks to original heterogeneity in the composition of the globe, and 

 instances the fact that the eruptive rocks west of the Rocky Mountains 

 exhibit certain peculiarities as a group. The remarkable difference, 

 however, between the igneous rocks of the Atlantic and Pacific slopes 

 of America seems to point not to any primitive difference between 

 those regions of the earth's crust, but rather to the operation of great 

 mountain-building forces, viz., those which produced the mountain- 

 axis which so sharply divides the two regions. How such forces 

 may act as promoting differentiation is doubtless a complicated 

 problem, but one way is very clearly indicated by such researches as 

 those of Mr. Barrow on the igneous gneisses and pegmatites of the 

 south-eastern Highlands of Scotland. 



Modern Crystal-gazing. 



Crystallography is scarcely a popular science ; but that it can 

 be made both interesting and intelligible, even to a Royal Institution 

 audience, was admirably shown by Professor Miers' three afternoon 

 lectures entitled " Some Secrets of Crystals." The first lecture opt-ned 

 with a contrast between the superstitions current in early times 

 respecting the origin and medicinal virtues of crystals — superstitions 

 which indeed still partly survive amongst the quasi-scientific, — and 



