6 Weed and Plrsson — Highwood Mts. Laccoliths. 



At the other end the relations are less clear, the valley wall 

 has been broken down and eroded somewhat so that there is 

 not the same vertical cut, but the exposures are quite sufficient 

 to show that the structure is less typical, the beds having 

 yielded to some extent by rupturing. The laccolith, however, 

 frays out in thin, fringing sheets, and the basal sheet, to which 

 it thins down, has the same thickness and character as on the 

 other side and may be traced, always at the same level horizon, 

 among the sandstones several miles up the valley. The per- 

 sistency of these basal sheets or indeed, more correctly, of the 

 one thin sheet of which ■ the laccolith is itself a greatly thick- 

 ened portion, is quite remarkable. 



The Laccolith Rock. — The rock composing the outer fring- 

 ing sheets is dark colored of dense texture spotted with 

 crystals of a black augite and round white spots of an altered 

 leucite ; it may be called a leucite basalt. The lower 12 to 15. 

 feet of the great columns of the main body of the laccolith are 

 also composed of this rock, which then passes into a fully gran- 

 ular one of granitic texture consisting of augite, olivine, 

 biotite and orthoclase, the shonkinite described in our paper 

 on Square Butte previously referred to. At the top the col- 

 umns pass into the porphyritic leucite basalt again, and as this 

 contact modification has a somewhat different color from the 

 dark shonkinite, each of the vast columns, at a distance, has a 

 quite distinct capital and base of a different color from the 

 main body of the shaft. 



Interior of the Laccolith. 



It has been mentioned that the central portion of the lacco- 

 lith wall has been broken by an intersecting stream gorge 

 which has cut down through it and into the underlying sand- 

 stones upon which the laccolith rests. A small stream of 

 clear water issues out of the gorge and nourishes some vegeta- 

 tion in its bottom, whose greenness is in grateful contrast to 

 the somber rock masses rising on all sides and to the arid valley 

 of Flat Creek. At the very entrance to the gorge and pic- 

 turesquely shaded by a grove of tall cottonwood trees stand the 

 buildings of the Wilson ranch looking down the slope into the 

 Flat Creek valley or Shonkin Sag, as seen in fig. 4. Follow- 

 ing up the stream, its bottom gradually rises above the sand- 

 stones until it is in the igneous rock and at the same time it 

 branches out into tributary gulches, so that in this way the 

 whole interior of the laccolith is thoroughly dissected and laid 

 bare for inspection and study. 



In the rising floor of the gulch, one soon comes to the con- 

 tact of the igneous rock and the sandstones on which it rests, 



