62 Canadian Record of Science. 



deposit would form the base of a volcanic cone, built up 

 upon it by later eruptions of ashes and lava. Judging 

 from the composition of the rocks of Mount Boyal this 

 ash would consist of a nepheline bearing lava, which 

 in its finely divided state would readily alter by the action 

 of percolating waters, to the impure dolomite now forming 

 the matrix of the breccia. This seems a reasonable in- 

 ference, as the nepheline bearing dyke rocks of the region 

 show a marked tendency to alter to rhombohedral mag- 

 nesian carbonates. 



Jaggar's explanation of the breccia at Mato Teepee as 

 a lava filled with rock fragments, constituting part of a 

 sheet extending laterally through the strata, does not 

 seem to be applicable in the case of the Montreal occur- 

 rences. 



With respect to the position of the breccia on the Utica 

 and other Ordovician formations, two explanations seem 

 possible. 



The first is, that deep fissures were formed during the 

 first stages of the eruption, and that these extended down 

 to the several formations of the Ordovician, and were filled 

 wholly or partially by volcanic breccia. Figure 4 is a 

 cross section from Mount Eoyal to St. Helen's Island. 

 The line C. I), gives a profile of the volcano, and at the 

 right of the section is a fissure extending down to the 

 level of St. Helen's Island. In the other localities the 

 fissures would extend down to the Calciferous or Trenton 

 Subsequent erosion has reduced the surface from C. D. 

 to A. B. as it is at present. 



The second explanation demands a period of erosion 

 after the deposition of the lower Oriskaivy, which would 

 reduce the land surface to E. F. as in figure 5. Here it is 

 seen that small remnants of the Lower Helderberg, and 

 Lower Oriskany, remain directly over the centre of the 

 subsequent volcanic activity. These and parts of the 

 other formations, would be thrown out and deposited on 



