16 APPENDIX. 



shall content myself with describing them once for all. 

 Moreover, to facilitate the reference which it may be neces- 

 sary to make to the different geological divisions of a group 

 of rocks which I propose to consider under the name of 

 Dixon's Group, or Dixon's Bluff, I shall note the divisions 

 of this group, in their ascending order, by the letters of the 

 alphabet, viz. : — 



" A. .'Vrgillaceous limestone, containing inoceramus bara- 

 bini, in great number and very much compressed, and so 

 arranged as to give the rock a slaty structure. This stratum 

 sinks below the bed of the river, and, consequently, its thick- 

 ness is indeterminable ; that part of it above the water on the 

 day of my examination was three feet. Starting from this 

 place, and ascending the river, this rock must necessarily 

 disappear below the level of the water. It is, probably, more 

 conspicuous in the two preceding cliffs I have referred to 

 before, but which I had not an opportunity of examining. 

 The upper portions of the rock, that I did examine, contain 

 nodules of iron pyrites, bemg an assemblage of small cubic, 

 cubo-octaedral, and octaedral crystals. 



" B. A calcareous marl, generally from thirty to forty feet 

 thick, but, at this spot, reduced, by a slide, to fifteen or twenty 

 feet. Its colors are grey, greyish-blue, and sometimes yel- 

 low. It contains but very few fossils. I found, myself, but 

 one orbicula, and what appears to be a fish-scale. 



" C. This is a slightly ferruginous clay-bank, of a yellow- 

 ish color, with seams of selenite, and affording, occasionally, 

 rounded masses, somewhat resembling septariae. The sele- 

 nite is in acicular crystals, or in its more usual form of rhombic 

 prisms, variously truncated. 



Such are the three divisions that I have thought necessary 

 to make in this group of rocks, and which are always thus 

 associated as the river is ascended. This group is the basis 



