592 . REPORT- -1002. 



each other by intermolecular spaces, and have their modes of motiou, seeins 

 essential to the comprehension of rock metamorphism. 



The important experiments of Sir W. Roberts-Austen on tlie difiusion of gold 

 in pure lead throw considerable light on this subject. 



Disks of solid gold were held against the bases of cylinders of lead by clamps, 

 and were kept in an upright position at the ordinary temperature for four years. 

 At the end of this time it was found that the gold had difthsed upwards in the 

 solid lead, for a distance of 7"65 mm., in sufficient quantity to be detected by the 

 ordinary methods adopted by assayers. Traces of gold were found still higher. 



"When a column of molten lead, 16 cm. high, was placed above solid gold and 

 kept at a mean temperature of 49:^° C, that is to say, at 166° above the melting 

 point of lead, but 509'7° below that of gold, the gold diffused in considerable 

 amount, to the top of the lead column, in a single day. 



Sir W. E.oberts-Austen"s experiments, above alluded to, demonstrate that even 

 such metals as gold, whose melting point is as high as 1061*7° C, exhibit a 

 measurable amount of kinetic energy at the ordinary temperature and pressure. 

 Great results may no doubt be brought about at ordinary temperatures and pres- 

 sures, when time, as in the laboratory of nature, is practically unlimited ; never- 

 theless the importance of high temperature and high pressure, in operations 

 connected with metamorphism, can hardly be overrated. 



Not only does a rise in temperature increase the energy of the chemical actions 

 and reactions which produce the mineralogical changes embraced by the term 

 metamorphism, but it increases the porosity of minerals and facilitates the passage 

 of liquids and gases through their pores. 



The cohesion of molecules is lessened, the amplitudes of their vibrations, 

 rotatory or other movements, are increased, and a passage is opened for the advance 

 of chemical materials into the heart of the crystal. 



Increase of temperature thus not only throws open the doors of the mineral 

 fortress attacked, but gives enhanced energy to the invaders. The fact that the 

 mineral components of a rock are, under conditions of heat and pressure practically 

 porous to heated Avater, laden with chemical reagents in soliition, is frequently 

 brought home to the mind of the petrologist in a very tangible way. "\Ve some- 

 times observe, for instance, that metamorphic changes begin at the heart of a 

 crystal, and leave the peripheral portions of it fresh and unaltered. 



In such cases the chemical agents of change have evidently passed freely 

 through the outer parts of the crystal, and have by preference selected its internal 

 parts for attack. 



In order to explain clearly how this remarkable result takes place, in the cases 

 i-eferred to, it will be necessary to diverge for a few minutes to consider another 

 branch of our subject. It is diihcult, if not impossible, to lay down any hard and 

 fast rule of universal ajiplication, because the conditions under which igneous 

 rocks crystallise vary with temperature, pressure, the relative proportion of con- 

 stituents, and other local causes, and these variations in the conditions may 

 materially atl'ect the results; but I think the rule that minerals cry.stallise out of 

 a molten magma in the order of their basicity is of very frequent if not of abso- 

 lutely general application. This rule also governs the growtli of individual 

 crystals, especially those that exhibit what is known as zonal structure. Take, 

 for instance, the felspars of an igneous rock. A gradual passage may ftequently 

 be traced by the petrologist fiom one species of felspar at the heart of a crystal 

 to another distinct species at its periphery. Sometimes a crystal is made up of 

 more than two s])ecies, which shade more or less gradually into each other. In 

 accordance with the rule laid down above, the more basic species formed first •, 

 then, as the percentage of the bases left in the magma gradually decreased, owing 

 to tlie first formed crystals having taken a lion's share of the a^ ailable bases, the 

 ff-lspars that formed later became gradually more and more acid in composition. 

 Thus a large felspar of slow and gradual growth may be composed of several 

 zones, each zone being successively less basic and more acid than that upon which 

 it crystallised, each successive zone thus j)0ssessing slightly different physical 

 properties from the one that formed before it. These statements are capable of 



