CHARACTER AND DISTRIBUTION OF THE ROCKS 255 



south, range 42 east. On the other hand, a ledge of rhyolite about 500 

 feet south of the center of the same section, on the north slope of Middle 

 Cone, contains inclusions of basalt. Thus some of the rhyolite is younger 

 than some of the basalt. Similar evidence is borne by the occurrence of 

 rhyolitie ash above basaltic debris at several localities. 



Olivine Basalt 



The olivine basalt is the most widely distributed of the igneous rocks 

 of the region. The Lanes Creek Quadrangle contains a number of sepa- 

 rated areas of basalt, some of which may be connected under cover, but 

 this can not safely be assumed. One of the larger areas is continuous 

 with the Henry Quadrangle basalt, which, like a somber sea, separates 

 mountainous sedimentary masses near the east and west borders of that 

 quadrangle and surrounds the three rhyolitie cones south of the Black- 

 foot Eiver Eeservoir. The basalt is partially overspread in some places 

 by hillwash and alluvium and elsewhere by soil and deposits of basaltic 

 and rhyolitie ash. Locally these deposits have a thickness as great as 40 

 feet or more. This is particularly the case in districts south and west of 

 the reservoir. The outcrops of basalt, however, are so numerous and so 

 well distributed as to leave no reasonable doubt of the continuity of the 

 rock-mass. Hence that district is all mapped as basalt. 



More than half of the Cranes Flat Quadrangle is underlain by basalt, 

 which is distributed in much the same way as in the Henry Quadrangle. 

 The basalt forms two great areas nearly separated by the high sedi- 

 mentary ridge that extends northwestward through the center of the 

 quadrangle. 



The basalt occurs in four general forms, namely, flows, cones or craters, 

 dikes, and ash beds. 



The great body of the basaltic rocks of the region is in the form of 

 flows. In some places, as shown by well records, sedimentary deposits lie 

 between the flows. In most places, however, nothing intervenes between 

 the flows ; their distinction is made on textural differences, which in turn 

 have induced differences in weathering. Thus cliffs of massive basalt 10 

 to 20 feet high are locally separated by thin, platy basalt more or less 

 concealed by blocky talus. The basalt has the characteristic columnar 

 and cross- jointing with "ball-and-socket" weathering. The number of 

 flows recognized in any one vertical section is usually not more than two 

 or three, but the margins of flows form low cliffs on the broad basaltic 

 areas; so that from favorable points of view the flows resemble dark 

 waves, such as on a smaller scale are formed by successive sheets of water 

 outpoured on a surface and then frozen. 



