10 



Weed and Pirsson — Highioood Jits. Laccoliths. 



the laccolith. Evidences of scouring and polishing were not 

 observed on those parts of the laccolith surface which were 

 crossed, but it is probable that they may exist. The dense hard 

 basalt of the contact variety was noted as scratched and striated 

 boulders in the drift. At the close of the glacial period the 



river retreated to its present 

 position, and since then erosion 

 has deepened and extended the 

 dissection. The relative posi- 

 tion of the former Missouri val- 

 ley, the laccolith, and the dis- 

 secting stream are shown in the 

 adjoining fig. 7, which repre- 

 sents them in a diagrammatic 

 ground plan. In fig. 8 is given 

 a stereogram of the laccolith in 

 which the various points men- 

 tioned are shown in a diagram- 

 matic way. The dotted surface 

 at the top represents in a gene- 

 ral manner the area bared by erosion. 



Fig. 7. AA, former Missouri valley 

 BB, laccolith. CC, intersecting tribu- 

 tary. 



Internal Structure. 



From the description which has been given, it is clear that 

 the outer portion of the laccolith is different from the interior 



Fig. 



Stereogram of Shonkin Sag laccolith. 



in the character and types of rocks composing it. These may 

 be concisely summed up in the following section, which may 

 be regarded as approximately correct. 



