LOWEST HED SANDSTOiNE. 397 



sometimes happens that a gradation takes place 

 at the junction . The argillaceous schist becomes 

 coarse and filled with fragments, forming that 

 which is called gray wacke ; and as a similar cha- 

 racter is found in the first beds of the sandstone, 

 the boundary between the two rocks, and, conse- 

 quently, that between the primary and secondary 

 strata, is indefinite. The peculiar interest at- 

 tached to this occurrence, is the province of the 

 geologist. The mineral distinction cannot, of 

 course, be made ; and the specimens must neces- 

 sarily, as in all similar gradations between ap- 

 proximate strata, be considered as of ati interme- 

 diate nature. 



mli The substances which, as already observed, 

 are found to occur in alternation with the red 

 sandstone, are, generally, varieties of shale. A 

 gradation, in this case also, sometimes takes 

 place at the points of change ; so that interme- 

 diate rocks of no very definite character, are 

 sometimes found among the sandstone strata. 

 Uuder the same circumstances, small strata of 

 limestone also, are occasionally found ; and even 

 there become sometimes indefinite at their sur- 



