204 AU6TEALASIAN ANTARCTIC EXPEDITION. 



particular solvent) and lowers its melting point. If a stress is applied to a rock, solution 

 tends to occur at the points immediately under the stress with simultaneous deposition 

 in planes at right angles. If solution occurs on the ends of a prismatic crystal, for 

 example, and deposition occurs in the plane at right angles, the prism will be flattened 

 first into a lenticular aggregate and finally into broad flakes. By molecular displace- 

 ments in this way the rock can yield to a stress as if it were plastic, and the development 

 of a schistose structure can be pictured with the ordinary conception of solution. If 

 there appear new mineral forms, such as the platy minerals like biotite, which can be 

 stable against the stress, a completely recrystallised rock may arise. 



2. Solid Diffusion. 



In many cases, however, microscopic solution cannot be so readily pictured aa 

 the means of transport, as for example, in the development of large porphyroblastic 

 crystals in metamorphic rocks which have only been subjected to weak stress, when 

 the pressure is mainly hydrostatic. The growth in such cases may be uniform in all 

 directions. When a corona of garnet crystals develops during the recrystallisation of 

 dolerites, by the- reaction between felspar and augite, the garnet crystals must occupy 

 space formerly occupied by felspar and augite ; and this space is not provided by 

 solution as in metasomatic replacement. As the garnet crystal grows it draws its supplies 

 from adjacent regions, and in this there is a molecular transfer of material and, at the 

 same time, a minute metamorphic differentiation. The larger the crystal grows the 

 greater must be the distance over which it draws its supplies, and this distance must 

 be appreciable when the garnet becomes over an inch in width. The well known 

 secondary enlargements of hornblende, augite, plagioclase, and orthoclase which have 

 been likened by Holland* to the growth of garnet, imply similar transfer. So also does 

 the growth of larger crystals at the expense of the smaller crystals, with its consequent 

 increase in grain size a fact that is well known among the crystalline schists. f 



A full discussion of the problem of solid diffusion is given by Desch in a report to 

 the British Association for the Advancement of Science. J It is pointed out that the 

 devitrification of glasses involves molecular diffusion in solids. As glassy rocks of 

 Palaeozoic age are unknown it becomes evident that solid diffiision is a process that has 

 been operative in geological time. It is shown that diffusion in metals has been estab- 

 lished beyond doubt. The cementation and decarburisation of iron, the segregation 

 and recrystallisation of constituents in solid metallic alloys are processes involving 

 true solid diffusion. 



In steels the iron carbide separating from solid solution is at first in a state of ultra- 

 microscopic division (troostite). On reheating and on different conditions of cooling 



* " Origin and Growth of Garnets," T. S. Holland, Kec. G.S.I., XXIX., p. 26. 

 f Op. cit., Grubenmann, vol. 1, pp. 39, 78. 



J " Roport on Diffusion in Solids," C. H. Desch, Brit. A.A.S., Dundee, 1912, p. 348. 



Troostite, Sorbite, and Peartite are not definite compounds but aggregates of ferrite (Fe) and cementite (Fe 4 C) with 

 different structures. 



