AN.NIVERSARY ADDRESS OF TUE PRESIDENT. ol 



is not impossible that some of the peculiar cases of zonal structure in 

 crystals may be due to changes of position, sometimes sli<?ht, durini>- 

 the process of formation. For instance, I think it extremely probable 

 that the zones of albite enclosing the large orthoclase crystals in the 

 "porphyroid" of Mairus in the Ardennes (I have no doubt this rock is 

 of igneous origin) are the records of two phases in its history. In 

 this way also very possibly the enclosure of hornblende by augite 

 crystals, or, vice versa, of nepheline by sodalite, &c., may be ex- 

 plained. 



Definite pressure also during the process of crystal-building 

 cannot fail to produce a marked effect. It may, I think, be taken 

 as an axiom that, creteris paribus, a molten mass under pressure will 

 crystallize more readily than one not so affected. It is very possible 

 that the devitrification of many of the ancient volcanic glasses has 

 been largely due to the pressure which they have undergone from 

 being buried deep below superincumbent strata. Molecular move- 

 ment within limits can take place in many substances long before 

 they cease to be solid, as is indicated, among other things, both by the 

 ordinary devitrification of glass and by Prof. Daubree's special expe- 

 riments ; and the mere fact that, in most cases, the specific gravity 

 of a substance is higher in a crystalline than when in a colloid state, 

 indicates the probable result of the application of pressure. But on 

 the present occasion I shall as far as possible avoid what may be 

 called subsequent metamorphosis, and confine myself mainly to 

 structures which are due to the application of a force definite in 

 direction during the process of crystallization. 



1. Crystals, already formed, wdll be arranged with their longer 

 axes in the direction of a tension, or at right angles to a pressure. 

 This, as every one knows, is the explanation of flow-structure in 

 microliths, and it is sometimes exemplified in the case of larger 

 crystals. 



2. Crystals, when forming, if exposed to a tension or pressure, 

 will develope with their longer diameters in the direction of the 

 tension, or at right angles to the pressure. This is especially well 

 exhibited by platy minerals, such as mica and diaUage. The foliated 

 aspect of granites and gabbros near to their junction with a level 

 surface of stratified rock has often been noticed ; but as I observe 

 that in the newly awakened enthusiasm for subsequent pressure as 

 an agent in modifying rock- structures there is some danger of 

 these being overlooked, I shall venture to recall a few from my own 

 experience. I have often noticed that a mass of granite intrusive 

 into a bedded rock has, for a depth of several inches, its mica-plates 

 parallel with the surface of junction, and without the slightest sign 

 of crushing. The most remarkable instance which I have ever seen 

 was in the neighbourhood of Bergen ; there a vein of granite, rather 

 more than a foot thick, threw off" a band some three inches wide 

 into a transverse fracture in the schist. It was obvious that the 

 angles of the latter rock, one being about 60°, the other about 120°, 

 would offer resistances definite in direction to the viscid mass of the 

 granite. Accordingly the plates of mica in the latter (as usual, not 

 numerous) were arranged perpendicularly to the normals to the 



