REPORT OF TEE CHIEF ASTRONOMER 349 



SESSIONAL PAPER No. 25a 



The chief interest of this body lies in the fact that it bears very numerous 

 basic segregations which appear to be themselves differentiates from the 

 magma from which the granite porphyry crystallized. The segregations are 

 round, abo\it one foot in maximum diameter, and seemingly quite uniform in 

 composition. They are of a dark greenish-gray colour, fine-grained to compact, 

 and in the field have the appearance of fragments torn off the volcanic forma- 

 tion close by. This was, indeed, the tentative field interpretation, although 

 it was there recognized that these small masses had also all the characteristics 

 of basic segregations. Microscopic study showed that the latter view is probably 

 the correct, one. The rock is somewhat porphyritie. with much altered pheno- 

 crysts of orthoclase and some of an oligoclase. No femic phenocryst was to 

 be seen. The ground-mass is a mass of minute, idiomorphic green hornblende 

 prisms embedded in small, interstitial crystals of orthoclase. with which a 

 few soda-lime feldspars may be mixed. A little titanite, less magnetite, and 

 abundant apatite in very minute prisms are the accessories. Chlorite, epidote, 

 and kaolin are the chief secondary minerals. These segregations have, thus. 

 the composition and most of the structural features of typical vogesite. Their 

 rounded and embayed outlines suggest magmatic resorption. They seem 

 to represent a lamprophyric derivative of the granodiorite and the almost 

 aplitic granite porphyry in which they lie would, on that view, correspond 

 to the other pole of the differentiation. More study needs to be given to this 

 case but it is worth while to point out this locality as an easily accessible and 

 perhaps fruitful one where magmatic differentiation in place may be discussed. 



Shatter-'belt. — The Trail batholith illustrates on a great scale the mechani- 

 cal disturbances which are so characteristically produced in country-rocks by 

 the intrusion of stocks and batholiths. The shatter-belt is not only unusually 

 broad but it is finely displayed along the eastern side of the Columbia river. 

 (Plate 33 and map sheet No. S.) This belt has been briefly described in the 

 American Journal of Science (Vol. 16, 1003, p. 123), and will be again referred 

 to in the following theoretical chapter (XXVI.) on the mechanics of igneous 

 intrusion. 



So far as observed within the ten-mile belt, the exomorphic influence of 

 the Trail batholith is more strikingly evident in the form of mechanical disrup- 

 tion than in the way of recrystallizing the invaded rocks. One reason for this 

 is that it is practically impossible to distinguish the thermal effects of the 

 intrusion from those produced by the heavy regional metamorphism which had 

 previously affected the traps. Yet it is probable that the strong schistosity 

 and the present composition of the hornblende-biotite gneisses and schists 

 (amphibolites) adjoining the granodiorite to the west of Sayward are, in largest 

 part, inhei'itances of the contact-metamorphism. 



Near the Columbia river contact north of Sayward. the batholith is cut 

 by dikes of monzonite porphyry and of camptonite. On the railway west of 

 Trail it is cut by dikes of hornblende-biotite gabbro. 



