Weed and Pirsson — Highioood Mts. Laccoliths. 9 



marked by thick pillars of the transition rock. This is merely 

 an earlier stage of the development of the isolated mushroom- 

 like masses, and one phase of it is shown in fig. 6. 



The thickness of theplaty white syenite is about 25 to 30 

 feet and above, always ascending, it passes in narrow limits 

 into the same coarse, crumbly, transition rock as that below it. 

 Here on top, however, this is but about five feet in thickness 

 and it then passes into a coarse shonkinite like that below ; 

 this in about five feet begins to 

 be denser and blacker, and in 

 about five feet more it becomes 

 a porphyritic rock spotted with 

 augites and altered leucites, the 

 same leu cite basalt as at the very 

 bottom. 



We are now on the very top of 

 the laccolith ; an elevated plateau 

 devoid of vegetation save for a 

 sparse growth of grass. On all 

 sides, except on the cliff looking 

 down into .the Shonkin Sag, it rolls gently down towards the 

 sediments showing a marked turtle-back form and its center 

 is cut into by the descending gulches which merge into the 

 dissecting gorge. Except around this basin it is covered with 

 the overlying sediments, but here the top of the igneous rock 

 is exposed. In one place the cover was found to be cracked 

 and filled with a dike of much altered basic rock. 



Early stage of erosion 



Cause of the Dissection. 



In the foregoing description there has b,een given somewhat 

 fully the actual facts to be observed in the field, and there now 

 remains the interpretation of them. From what is stated else- 

 where in the chapter on the Shonkin Sag and from the pres- 

 ence and character of the morainal drift, it is evident that dur- 

 ing the glacial period the continental ice sheet in this region 

 pushed its way as far south as the lower slopes and foot-hills 

 of the Highwood Mountains. In its advance the Missouri 

 River was driven southward and forced to flow around the 

 foot of the ice front. In doing so it excavated the valley of 

 the Shonkin Sag and thus sawed down and through the outer 

 circumference of the laccolith as a knife might cut down 

 through a cake. It is possible and even probable that the 

 river deepened and cut out an already existent smaller valley, 

 entirely removing the outer section of the laccolith. The 

 position of the intersecting tributary was also previously deter- 

 mined, and the ice moved over the already eroded surface of 



