STRUCTURAL DETAILS 



313 



phase of volcanic activity. Both sedimentary and igneous fragments are 

 often rounded and appear to have been churned up togetlier. Distinctly 

 bomb-like masses of lava are often seen. The sedimentary fragments 

 vary in size up to three feet in diameter, and are frequently much baked 

 on tlieir outer surfaces. As a nde, no evidence of stratification is visible 

 in the agglomerate, but occasionally distinct traces of rude bedding are 

 made out near the side of the bntte, as if during a somewhat quieter 

 phase of the eruption an old conduit Avere filled up witli the fragmental 

 material which was not carried clear away from the exit. 



Up through the great mass of agglomerate came 'the basaltic lava, ap- 

 parently rising higher than the agglomerate in places and flowing over 

 it. How much higher the lava reached is problematical. On the south- 

 eastern side is seen a splendid contact between the agglomerate and the 

 lava, the line of contact running up the face of the butte to near the top, 



FiGUUE 5. — Butte number l/, hIiuii-'ukj lioi-izoiiial ficilimfiit.v siirraiiniliii!/ i</iivuii.^ Cuic. 

 with vertical Contact exposed on south Skh: 



and then becoming liorizontal where the lava flowed out over the ag- 

 glomerate, while the columns, being at right angles to the contact, are 

 horizontal so long as the contact is vertical, but change to Aertical where 

 the contact is horizontal. In other places the columns are less regidar. 



At various points around this Initte the sandstone was found in phiee 

 well up tlie sides of the neck, while in the ravine the igneous rock could 

 be followed down to much lower levels, leaving no doul)t in the niiiul nf 

 the observer that the contact was essentially vertical, and that the sedi- 

 ments must formerly have enclosed the rudely cylindrical butte on all 

 sides, having since been partly removed by erosion. 



It should be noted that this butte and its enclosing sediments are found 

 several hundred feet lower, both as to actual present elevation and as to 

 stratigraphic position, than the Cabezon and Twin Peak buttes. There 

 is seen to be no relation between the buttes. so far as absolute or relative 

 elevations are concerned. Their heights and the part of the sedimentary 

 series still found about them depend on the amount of destruction accom- 

 plished by erosion at each particular point. 



