SON PLATEAU. 231 



groups. The slips and alternations of the rocks are too confused for 

 close description ; much of the ground is covered by debris chiefly of 

 the carboniferous group, projecting from beneath which, isolated por- 

 tions of any of the local rocks may be found. 



There is here very commonly a zone of sandy, olive, micaceous beds 50 

 feet or so in thickness at the base of the carboniferous formation, and 

 the strong limestones above may be estimated at 500 feet at 

 least, if not more, — the repeated step-cliffs making its depth appear 

 much larger than it really is. The lavender and grey clays again form 

 a thick band at the top of the " speckled sandstone " group ; they are 

 gypseous, and in places contain light olive, micaceous sandstone layers, 

 ferruginous beds, and layers of black and grey shale. The thickness 

 of this portion of the group varies from 50 to more than 100 feet, 

 and the underlying red, white, and speckled sandstones and clays are 

 from 250 to 800 feet. Between these and the purple sandstones below 

 there is still sometimes an eight-feet greenish shaly zone on the silurian 

 horizon ; and in the red marl beneath, thin purple sandstone and grey 

 gypseous and cherty-looking dolomitic flaggy layers occur. 



Great masses of white clay occur in the debris on the end of the 

 Jabi spur above the left side of the Dokri gorge. 



Jabi spur. 



Some disintegrating nummulitic limestone near 



the place suggests that the clay may result from the decay and re-arrange- 

 ment of its lower marly beds, but the local debris conceals the relations. 



About a mile north of Jabi, portions of the carboniferous rocks. 

 Ammonite or Phyllo- slipped, fallen, and displaced, still retain sufficient 

 ccms.— (Waagen.) continuity of relation to enable their former place 



in this series to be recognised. In a broken-down mass, belonging to the 

 lower part of the upper beds of this carboniferous formation. Dr. Waagen 

 found the oldest known Ammmiite, or, as he has since determined it, Thyllo- 

 ceras j which unique and interesting fossil forms the subject of a short paper 

 in the Geological Survey Memoirs, Vol. IX, p. 351. It was associated in 

 the limestone with the following: At/i^ris Ropsii, Productus costatus^ 



( 2^1 ) 



