396 DESORIPTIVP] GEOLOGY. 



and downthrow of the whole mass to the jaorth of the Hne of this canon, 

 where the western member of the fold is lost beneath the plains. 



The Archaean rocks, as exposed at the mouth of Ogden Canon, con- 

 sist of a varying series of hornblendic and micaceous gneisses, having a 

 strike to the north 20° west, and a dip to the westward, and of a granitoid 

 mass, in which no structure is observable. They present much that is 

 worthy of special interest and study, in their gradual transitions from a 

 compact to a decidedly schistose structure. As seen, on a large scale in 

 the field, the bedding is distinctly marked by darker or lighter bands, char- 

 acterized by the presence of either hornblende or mica as the prevailing 

 constituent. Again, the beds are frequently marked by irregular, nar- 

 row bands of limpid quartz, or by nearly pure feldspar layers, usually 

 accompanied by a little mica, while the rock masses also carry segre- 

 gations of milky- white quartz, and in some places of dark green horn- 

 blende. In a hand specimen, many of the rocks develop a decidedly 

 granitic structure, breaking with a sharp, angular fracture like a compact 

 granite, but the gneissic tendency is, however, apparent in the general 

 airangement of the fibrous hornblendes in a direction parallel with the 

 bedding of the rock. A typical specimen of the finer-grained masses is of 

 a dark-greenish color, with a somewhat rough texture, and characterized by 

 the presence of both hornblende and mica in considerable quantities. It is 

 made up of minute crystals of delicate red orthoclase and white plagioclase 

 feldspars, dark-green fibrous hornblende, with flakes of brown biotite, while 

 quartz is present in subordinate amounts. With the magnifying-glass, par- 

 ticles are seen of what is apparently mica, with a distinctly yellowish tinge, 

 and under the microscope considerable apatite has been detected, and what 

 is supposed to be white mica. This rock passes into a coarser-grained 

 variety, where the hornblende and mica are found less completely blended, 

 and the feldspar plays a more important part. The rock with the least 

 gneissic structure, from this locality, is a moderately fine-grained mass, 

 composed chiefly of quartz, feldspar, and hornblende. White vitreous plagio- 

 clase is very abundant and intimately mixed with grains of quartz. Dark, 

 shining plates of mica are present in small amounts. Another similar speci- 

 men, in a position not far from this, shows more of a tendency to the banded 



