Weed and Pirsson — Highivood Mts. Laccoliths. 5 



Ends of the Laccolith Wall. — From what has been said 

 above, it is clear that this cliff wall is the cross section of a 

 flat mass intruded into and lying between the beds of sand- 

 stone, its laccolithic character being seen only at the outer 

 borders. No real distinction can be drawn which will sepa- 

 rate laccoliths from intrusive sheets : the most typical laccolith 

 can be regarded as a very short, thick, domed sheet, and con- 

 versely, an intrusive sheet can be regarded as a very flat, thin 

 laccolith. In our work in the mountain areas of central Mon- 

 tana, we have observed all degrees of transition between the 

 two. Nevertheless, in the typical laccolith we presuppose a 

 certain, quick up-arching of the strata which we do not observe 

 in the typical intrusive sheet. 



Fig. 3. Sketch of east end of laccolith showing relations of strata and 

 intruded sheets. 



This character is well shown by the Shonkin laccolith at the 

 eastern end. The main body of the laccolith rounds quickly 

 down at a pretty sharp angle until it becomes only the thin 

 intrusive sheet, previously mentioned, not more than ten feet 

 in thickness, which on the same horizon extends a great dis- 

 tance out into the sandstones. Besides this main lower fring- 

 ing sheet there are two or three lesser ones which do not 

 extend far and quickly die away in the sedimentary beds, and 

 in addition, at the spring of the demi-arch of sandstones, where 

 there must have been a certain relief of pressure, there are 

 small subordinate intruded masses of igneous material of 

 lenticular character and somewhat cusp-shaped cross section. 

 These relations are shown in fig. 3 from a careful outline draw- 

 ing made on the spot, and are also seen in the photograph, 

 fig. 2. All of these details are as clearly seen on the great 

 cliff wall of the valley as in the diagrammatic section, the con- 

 trast of the black intruded rock masses with the light-colored 

 sandstones being of the most pronounced kind. It is notice- 

 able to see how the beds have adjusted themselves to the 

 upward bend without much breaking or rupturing. 



