REPORT OF THE CHIEF ASTRONOMER 175 



SESSIONAL PAPER No. 25a 



In the section northeast of Eldon the Bow River rocks are not well exposed 

 but the limestone shows about 3,500 feet of its thickness. The dip averages 

 25° northeast. At its base the limestone is rather massive and is composed of 

 firmly knit, thin beds of alternating', gray-weathering and light brown to buff 

 weathering, impure carbonate. The gray layers carry little magnesium car- 

 bonate, but the brown-buff layers are dolomitic. The thickness of these layers 

 runs from a fraction of an inch to two inches. About 1,000 feet above the base 

 a band, 100 feet or more in thickness, is exceptional in being thin-bedded and 

 easily cleaved but it preserves the buff weather-tint and a dolomitic composi- 

 tion. Similar thin zones occur both above and below this band.. In general, 

 however, the limestone is not only thick-platy in structure as in the coursing 

 of heavy masonry, but, like the Siyeh limestone, shows a rather uniform, buff 

 to brown weathering tint and high content of magnesium carbonate. Both 

 phases of the fresh limestone are normally rather dark-gray or bluish-gray- 

 The rock is often arenaceous or argillaceous; the weathered surface is rough- 

 ened very often, through the projection of the sand-grains. The unequal dis- 

 tribution of carbonate and impurity renders the surface characteristically 

 pitted. 



In a specially argillaceous bed at the top of the 100-foot, thin-bedded lime- 

 stone band, fossils, chiefly trilobite fragments of apparently Middle Cambrian 

 age were discovered. Middle Cambrian fossils were also found at the base of 

 the whole limestone formation. 



The likewise excellent section above Laggan, thirteen miles to the north- 

 westward and across Bow river valley, disclosed practically identical features 

 in the limestone. The lower beds, which were found to crop out at Lake Agnes 

 afforded fragments of indeterminable trilobites and crustacean tracks. 



Perhaps the most significant fact derived from the section is that the lime- 

 stone very often possesses the typical molar-tooth structure so characteristic of- 

 the Siyeh limestone. (Plate 22). 



This structure is not well developed in any part of the Eldon section but 

 it was again seen in the limestone at Mt. Stephen and at several other points 

 along the railroad, where the line cuts across outcrops of the Castle Mountain 

 formation. As in the Siyeh limestone the molar-tooth structure seems to be 

 best developed where- the rock has been locally cleaved or cracked by orogenic 

 stress. 



The thick quartzite underlying the limestone is well exposed at the Laggan 

 section. It is a thick-bedded formation, heavy plates of quartzite alternating 

 with subordinate, thin, fissile intercalations of silicious metargillite. The 

 general colour of the quartzite on a fresh fracture is pale reddish to reddish- 

 gray; the colours of the weathered rock are in general, rusty-brown and red 

 but vary through white, pale gray, pale red, pink, brown, purple, and, 

 toward the top of the formation, deep maroon-red. The argillaceous interbeds 

 are dark-gray or greenish, weathering greenish or grayish-brown. Cross- 

 bedding, ripple-marks, annelide trails, and borings are all common. The litho- 

 logieal similarity of the lighter tinted and thicker beds to the Ripple^ quartzite 



