Vol. 4] Smith. — Upper Region of Main Waller River. 



5 



Sing-ats'-e Ridge. — This ridge at the northern end, within 

 the limits of the map PI. 1, is made up of andesite and andesite 

 tnff and breccia, similarly to the first ridge. The cap of later 

 andesite, however, in this case is broken up, and along with it 

 there is considerable of the breccia which is in the tuff; the 

 breccia, in fact, forms nearly half of the cap. This is undoubt- 

 edly due to the tuff being easily eroded and carried away from 

 around the blocks, thus leaving them residual. The only con- 

 tact between the rhyolite and the tuff was found here, and its 

 character is such that the relation of the rhyolite to the tuff 

 could not be made out satisfactorily. The tnff is fragile, and a 

 sharp contact was not to be expected. It shows no planes of 

 slipping, and no pieces of the rhyolite were found in the tuff to 

 indicate that there had been any faulting. The rhyolite appears 

 from beneath the tuff, and in this connection the composition of 

 the tuff is of some importance. The apparent absence of all 

 other rock fragments from the tuff except the hornblende ande- 

 site is worthy of note ; and perhaps of equal importance is the 

 composition of the tuff aside from the hornblende andesite brec- 

 cia it contains. There was found in it no quartz, but feldspar 

 and a little dark-coloi*ed ferrb-magnesian mineral were observed. 

 Some good cleavage flakes of the feldspar gave extinction angles 

 of andesite and labradorite, some flakes showing an optic axis 

 nearly normal. This indicates the tuff to be andesitic. The 

 material of the tuff is fine and splintery. The absence of all 

 other rocks just mentioned from the tuff will be considered in 

 another connection when the structure is discussed. The rhyo- 

 lite makes its appearance along this second ridge in considerable 

 force, and two dykes of rhyolite about four miles apart are quite 

 marked ; and along with one there occurs a dyke of the horn- 

 blende andesite. The two seem to mark the same line of frac- 

 ture. West of this dyke there is an area of andesite and tuff, 

 the latter showing some well stratified sandstone and some loose 

 blocks of conglomerate ; and midway of the ridge along the 

 lowest canon cutting across it an earlier intrusive is uncovered, 

 as weil as ^ome granite and hornblende andesite. The rhyolite 

 evidently formerly covered these areas. Near the south end of 

 the second dyke the hornblende andesite and the rhyolite reach 



