258 G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



not residual, so far as observed, but represent finely divided particles of 

 drab-colored clayey material so arranged as to suggest wind-blown dust. 

 Locally, the basalt is overlain by rhyolitic ash, hillwash, or alluvium. 



The thickness of the basalt varies considerably from place to place and 

 probably depends to some extent on the character of the underlying topog- 

 raphy. The maximum exposed thickness is about 250 feet, but the actual 

 maximum thickness is probably much greater. 



Contact Metamorphism 



No evidence of contact metamorphism has been observed in connection 

 with the basalt and practically none with the rhyolite. The andesitic sill 

 at Sugarloaf Mountain and some of the andesitic dikes farther northeast 

 have produced a certain degree of metamorphism in beds of the Homer 

 limestone (Lower Cretaceous?) with which they have come in contact. 

 The main effect has been the induration or crystallization of the lime- 

 stone for a few feet from the igneous rock. There has been some inter- 

 penetration of the limestone with silica and at two localities dark circular 

 masses of chert about one-eighth inch in diameter give the rock a peculiar 

 spotted appearance. These may be due to the replacement of poorly pre- 

 served tiny gastropods, such as occur in some beds of the limestone. 



In the northwest y^, section 9, township 6 south, range 43 east. Lanes 

 Creek Quadrangle, a metalliferous prospect in sandy limestone of the 

 Wells formation (Pennsylvanian) furnishes a number of minerals, sam- 

 ples of which have been examined by Mr. Larsen. Among these are yel- 

 lowish green garnet in masses and small crystals, c^uartz, chalcopyrite, 

 malachite (?), and tabular hematite. The mineralization is purely local 

 and points to the proximity of igneous rock, but none is exposed. 



Epochs of igneous Activity 



There is evidence for at least five epochs of igneous activity. The pos- 

 sibility of three other epochs is also recognized, though these may prove 

 to be identical with some of the five Just mentioned. 



(1) The early rhyolitic ash that now forms a poorly exposed indurated 

 bed in the Wayan formation probably represents a single volcanic out- 

 burst or, at best, a brief epoch of igneous activity. The source of the ash 

 is unknown, but probably lay west of the region here described. 



(2) The hornblende andesite porphyry represents probably the first 

 igneous activity within this actual region. The structural relations and 

 character of the rock at Sugarloaf Mountain show that considerable ero- 

 sion was necessary to produce the present exposures. This would mean 



