240 



The Cache Creek formation consists of very badly 

 metamorphosed sedimentary and eruptive material belong- 

 ing to the Main Pacific geosynclinal. The commonest 

 rock member is a cherty quartzite traversed by veinlets 

 of quartz. Dark massive argillites and contemporaneous 

 eruptives are of more local occurrence. Younger than the 

 above rocks, but in many places intimately interfolded 

 with them, is a limestone formation {Marble Canyon 

 limestone) now recrystallized to marble. Large foramini- 

 fers known as Loftusia columbiana and the diagnostic 

 Carboniferous fossil Fusulina are found in the Marble 

 Canyon limestone. Much of the gold found in the placer 

 workings along the Thompson and Fraser rivers may have 

 been derived from the Cache Creek quartz veins. On 

 account of the unfavourable character of the outcrops in 

 the railway section it has here proved impossible to ascer- 

 tain the full thickness. The estimate of Dawson is noted 

 in the foregoing table. 



The Nicola formation is well exposed in the Thompson 

 valley and consists of greenstones (altered eruptives of both 

 flow and fragmental type) intercalated with beds of 

 argillite and limestone. Crinoid remains, pelecypods, 

 terebratulas and pectens of several species are found in the 

 calcareous members of the formation. These fossils 

 place the series in the Triassic, grading up into the lower 

 Jurassic. G. M. Dawson estimated that the thickness of 

 the Nicola formation ranges from 10,000 to 15,000 feet. 

 The agglomerates and porphyrites of this formation, by 

 their much more metamorphosed and massive character, 

 are readily distinguished from those of the Tertiary. 



The batholiths, stocks and tongues which occur in the 

 district are referred to the upper Jurassic. They are made 

 up of granular intrusive rocks varying from granite to 

 granodiorite and diorite, and are all subalkalic in composi- 

 tion. 



During the Lower Cretaceous or late Jurassic, volcanic 

 eruptions broke forth along the east front of the Coast 

 range resulting in the accumulation of over 5,000 feet 

 (1500 m.) of acidic and intermediate lavas and tuffs — the 

 Spence's Bridge Volcanic group. This group has heretofore 

 been referred to the Miocene (Lower Volcanic group of 

 Dawson) but recently discovered plant, structural and 

 physiographic evidence place the group in the Lower 

 Cretaceous or late Jurassic. 



