BASAL CONGLOMERATES. 



85 





Sometimes the bowlders or cobblestones forming the conglomerate are 

 of considerable size, from one to four feet in diameter. One of the best 

 exposures of this character was seen in the canon of lower Rapid Creek, 

 and is illustrated in the annexed illustration. 



The large bowlders shown at the base of the section are perfectly water- 

 rounded and form a broad talus extending to the bed of the creek. They 



appear to have had no cementing material, but lie as 

 loosely as if they had been dumped from a cart. 

 This is apparently but a local accumulation; at a 

 short distance on either side no bowlders are visi- 

 ble, though the underlying slates are still exposed 

 to view. 



A similar heavy, conglomerate formation is found 

 in many other parts of the Hills, as, for instance, on 

 the headwaters of Box Elder Creek, where a bowlder 

 or cobblestone conglomerate with fragments two feet 

 or more in diameter, rests upon the upturned edges 

 of the Archsean quartzites and slates. 



In some places a conglomerate is found above 



Fig. 10.— Section of the Pots- x & 



clam sandstone on lower the base of the formation, in which the pebbles seem 



Kapid Creek, showing the L 



howhiers at its hase and t h ave been formed from previously existing portions 



its unconformity with the J- J ° x 



tidckness S °oi) h feet" T ° tal °^ tne sandstone. A good example may be seen near 



1. Massive gray limestone the eastern base of Terry Peak. Near Terry Peak 



(Carboniferous). 



2. Thin bedded impure lime- also Potsdam conglomerate was found so thoroughly 



stone (Carboniferous). 



3. Eeddish sandstone (Pots- metamorphosed that a fracture could be made as 



dam). 



a. Concealed. easily across the quartz pebbles and bowlders as in 



5. Coarse reddish sandstone J x L 



(Potsdam). any other direction, the silicious cement being- changed 



fi. Loose bowlders (Potsdam). J o o 



7. schists (Arch;ean). - n £ q Uar tzite as hard as the quartz pebbles themselves. 



The cementing material of the conglomerate is generally silex, and this 

 is reinforced by the oxide of iron. Carbonate of lime also has been found 

 in many localities, and in such cases the interstices are more perfectly filled 

 and the conglomerate is more compact. The calcareous .cement often con- 

 tains the remains of fossils and grains of glauconite. As has been observed, 



