REPORT OF TEE CEIEF ASTRONOMER 523 



SESSIONAL PAPER No. 25a 



From end to end of the body the igneous rock is profoundly altered; 

 hence it is almost impossible to ascertain its precise original composition. 

 It is now chiefly a mass of secondary minerals, including serpentine, talc, epi- 

 dote, zoisite, kaolin, chlorite, and quartz. A pale green actinolitic amphibole 

 never fails among the essential constituents; it also is probably secondary. A 

 darker-tinted green hornblende is commonly present, and may represent a product 

 of original crystallization from the magma. With this hornblende an original 

 plagioclase, probably labradorite, is usually associated. The feldspar is always 

 altered in high degree. A little magnetite, pyrite, and apatite, and much titanite 

 are accessories. 



The original rock was probably a basic diorite or a gabbro. It has been 

 greatly sheared and mashed and has degenerated into several secondary types. 

 The commonest of these is a massive greenstone bearing a fair amount of the 

 skeletal plagioclase and dark green hornblende which are regarded as primary 

 in origin. This rock is often intimately sheared and slickened, but is scarcely 

 a true schist. Toward the International Boundary the mass becomes distinctly 

 gneissic, with the field-habit of a medium- to coarse-grained hornblende-diorite 

 gneiss; under the microscope, however, this type was seen to be a hornblende- 

 zoisite-quartz schist, the amphibole being of actinolitic appearance. In certain 

 zones of specially intense shearing the rock has been converted into a garneti- 

 ferous talc-quartz schist. 



The amount of shearing and alteration undergone by this gabbroid intrusive 

 is of the same order as that seen in the Chilliwack volcanics, which have been 

 referred to the upper Carboniferous. It is possible that this great Vedder moun- 

 tain ' dike ' represents the intrusive phase of the same eruptions which gave rise 

 to the surface flows of the Chilliwack formation. In any case the greenstone is 

 certainly pre-Eocene and probably pre-Jurassic in age.* 



Custer Granite-gneiss. 



On Custer ridge, which locally forms the main divide of the Skagit range, 

 the Boundary belt crosses a considerable mass of crushed and now banded, intru- 

 sive granite. Its exposed area is known to be at least twenty square miles, but 

 it may be found to be much greater as the body is followed northward and south- 

 ward from the Boundary belt. The western limit of the banded granite so far 

 as mapped is fixed by the intrusive contact of the younger Chilliwack grano- 

 diorite. The eastern limit is fixed in part by a band of the Hozomeen sedi- 

 mentary series, into which the banded granite is intrusive; in other part, by the 

 very thick blanket of Skagit volcanics, which are clearly younger than the 

 gneissic granite. From its occurrence on Custer ridge this batholithic body may 

 be called the Custer granite-gneiss. 



* During the preliminary examination of this district, in 1901, the relations of 

 this schistose intrusive were not understood and the body was regarded as part of a 

 basal crystalline series. In the Summary Report for 1901 (page 51) this series was 

 given the provisional name ' Vedder Mountain gneisses/ The writer wishes to with- 

 draw this name which should obviously not be perpetuated in the literature. 



