464 MANSFIELD AND LARSENI NEPHELINE BASALT 



get sandstone, of Jurassic or Triassic age. Associated with the 

 nepheUne basalt and apparently surrounding it is a rhyolitic 

 rock that was not examined closely in the field but may underlie 

 or be penetrated by the basalt. This rock resembled other fine- 

 textured, siliceous, igneous rocks observed elsewhere in the res- 

 ervation and no sample was collected. The unusual character 

 of the accompanying basalt was not then recognized and the 

 contact of the two rocks was not observed. No fragments of the 

 rhyolitic rock were found in the basalt although some search for 

 them was made. Inclusions, at first mistaken for pieces of the 

 above-mentioned grit, proved to be fragments of a coarse-textured 

 igneous rock. 



Other igneous rocks occur in various parts of the reservation 

 but these all, so far as studied, are normal rhyolites, basalts, 

 andesites, and quartz-latites. In general the igneous rocks lie 

 around the borders of the sediments on the north, northeast, and 

 northwest, and compose larger or smaller portions of the lower 

 hills. They occupy some of the valleys and are accompanied in 

 places by large bodies of ash beds and tuffs. 



The geology of the igneous rocks has not been worked out in 

 detail and their geologic age has not yet been accurately deter- 

 mined. Present information points to at least four epochs of 

 volcanic activity, extending from perhaps middle Tertiary into 

 the Pleistocene. The order of succession appears to be an early 

 intermediate or basic series of eruptions with andesites and per- 

 haps basalt followed by outpourings of rhyolite. These were in 

 turn followed by olivine basalts in a number of places, well 

 exposed in the canyon of the Blackfoot River. The latest erup- 

 tions appear to have been latitic with both flows and elastics, the 

 last forming an extensive sandy area which is in part dune- 

 covered. The place of the nepheline basalt in this succession is 

 not known, but it appears to be extrusive and is very fresh. It 

 probably was not closely connected with the olivine basalts. 



PETROGRAPHY^ 



Description. The nepheline basalt is dark greenish gray and 

 is dense except for a few large, rough cavities. In the hand 



3 By Mr. Larsen. 



