Geology of St. Helen's Island. 61 



satisfactory hypothesis for the origin of one ought to 

 answer for the others. 



An analogy might be drawn between this breccia and 

 that of Mato Teepee in South Dakota. 1 Mato Teepee is an 

 erosion remnant of columnar trachyte, underlain by 

 horizontal strata of Jurassic age. On the side toward 

 the Little Missouri Buttes, which are about four and a 

 half miles to the north-west, there is an outcrop of breccai 

 beneath the porphyry. In this are found fragments of 

 rocks of various ages, some of which are not only strati- 

 graphically higher than the present position of the breccia, 

 but also higher than any of the surrounding formations. 

 These fragments of newer rocks are apparently from the 

 shales of Lower Cretaceous age, and the nearest occur- 

 rence of these in situ is at Little Missouri Buttes. 



Mato Teepee, according to Jaggar, is a remnant of an 

 old laccolith, the pipe through which the igneous material 

 rose being beneath the Little Missouri Buttes. The breccia 

 is really a decomposed porphyry, filled with fragments of 

 rocks from all the strata through which it forced its way. 

 The igneous material connecting Mato Teepee with the 

 Little Missouri Buttes, together with all the overlying 

 strata, has been removed by subsequent erosion, leaving 

 Mato Teepee and the breccia as isolated remnants. 



The St. Helen's Island breccia, like that of Mato Teepee, 

 contains fragments of rocks which are more recent than 

 the stratigraphical position of the breccia would lead one 

 to expect, and the following hypothesis is put forward to 

 account for its origin. 



When Mount Eoyal, which in all probability was once 

 an active volcano, was clearing its vent, a comminuted 

 lava or volcanic ash, together with fragments of all the 

 strata through which the lava forced its way, was thrown 

 out and deposited upon the surrounding country. This 



1. Jaggar, T. A., jr., The Laccoliths of the Black Hills U.S. Geol. Survey 1899-1900, 

 Part III. 



