282 PliOCEEDlXCS OF SECTION C. 



show independence. The occasion for such differences in neighbouring 

 volcanoes is to be sought in variation in the differentiation of niagmas 

 during the course of eruption from deep-seated to superficial positions. 



Among the results of such differentiation may be mentioned the 

 production of complementary rocks, which may occur in rock bodies 

 of various forms. A special case of local differentiation, usually 

 associated immediately with crystallisation and solidification of parts 

 of a magma, is the production of contemporaneous veins and 

 pegmatites. When complementaiy rock magmas are ei-upted so close 

 to one another in space and time that they come in conjunction while 

 still highly heated they may diffuse into one another, or blend to 

 such an extent as to yield hybrid rocks, or mixed dykes, sheets, &c., 

 as observed by Harker on the Isle of Skye. 



The eruption of rock magmas through solid rocks and their solidi- 

 fication in various positions wdthin or upon other rocks conditio!} the 

 modes of occurrence of igneous rocks, as those of lava streams, dykes, 

 sheets, laccoliths, &c. Aiid the parting or cracking of the solid rock, 

 uj)on cooling, or its arrangement after fragmentation in various ways, 

 leads to distinctive structures, such as columnar, spheroidal, brecciated, 

 and so on. 



Having acqizired a knowledge of the general principles applicable 

 to all igneous rocks, it is in order to consider more specifically those 

 occurring in all known j)arts of the world : first, systematically, accord- 

 ing to some comprehensive scheme of arrangement or classification, 

 and then acording to the groups, or associations, in which they occur 

 in various regions from which they have been described in considerable 

 detail ; that is, according to petrographical provinces, or comagmatic 

 regions. 



In order to describe many rocks a nomenclature is necessaiy, and 

 the confusion existing in that in present use is best understood by 

 considering the history of the growth of petrography, and the changes 

 that have gone on in the definition and use of the oldest and com- 

 monest rock names and descriptive terras. With this review should 

 be associated a sketch of the development of rock classification, which 

 has been furnished to the student in an interesting form by Cross. 



A successful treatment of the subject of igneous rocks along the 

 lines indicated would go far toward the removal of petrology from a 

 state of distracting empiricism, and the placing of it on a more 

 rational foundation. 



7.— THE KLONDIKE GOLD DISTRICT, IX THE YUKON 

 VALLEY, CANADA. 



By ROBERT BELL, I.S.O., M.D., D.Sc, LL.D., F.R.S., late Director of the Geolorjical 



Survey of Canada. 



Gold has been found in nearly every part of the Yukon Valley, 

 but in the following paper I shall confine my remarks to the Klondike 

 district, which has been surveyed and studied by officers of the 

 Geological Survey of Canada and has been inspected by myself, so 

 that I have the advantage of having seen what I will attempt to 

 describe. 



