General C. A. McMahon's Address. 463 



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 referred to, it will be necessary to diverge for a few 

 minutes to consider another branch of our subject. It is difficult, 

 if not impossible, to lay down any hard-and-fast rule of universal 

 application, because the conditions under which igneous rocks 

 crystallise vary with temperature, pressure, the relative proportion 

 of constituents, and other local causes, and these variations in the 

 conditions may materially affect the results ; but I think the rule 

 that minerals crystallise out of a molten magma in the order of their 

 basicity is of very frequent if not of absolutely general application. 

 This rule also governs the growth 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 

 frequently be traced by the petrologist from 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 species, 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 the first formed crystals having taken a lion's share of 

 the available bases, the felspars 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 possessing slightly different 

 physical properties from the one that formed before it. These 

 statements are capable of proof "When sections of felspars, such 

 as occur in thin slices of igneous rock, are examined under the 

 microscope in polarised light, petrologists can distinguish one species 

 from the other — when the direction in which the sections were cut 

 is approximately known — by measuring the angles at which they 

 extinguish from the twinning or the pinacoidal plane. 



This is not mere theory. Each species of felspar has its own 

 angle of extinction and its own index of refraction. The deter- 

 mination of these two factors enables a petrologist to prove optically 

 the change in composition ; or, in other words, the change in species 

 which has taken place in the successive zones during the gradual 

 growth of a large zonal felspar. 



Another general rule must now be mentioned. I think it may 

 safely be asserted as a broad rule, that the different species of felspars 

 are attackable by the chemical reagents which make themselves felt 

 in metamorphic action, in the order of their basicity ; that is to say, 

 the more basic felspars are more easily attacked than the acid ones. 

 When we bear in mind the facts stated above, we shall, I think, be 

 able to see clearly how it is that the peripheral portions of large 

 felspars in igneous rocks sometimes escape alteration, whilst the 

 cores of these crystals are converted into secondary minerals, such as 



