72 Harris and Crawford : 



(d) Summary. 



For the sake of clearness we shall recapitulate our results: — 



(1) The Riddell Grits are a series of sandstones, grits, mud- 

 stones, and shales, the shales being typically more rubbly than the 

 common Ordovician graptolite shales, and differing in colour 

 and texture. 



(2) The associated and interstratified shales contain Diplo- 

 graptus and Climacograptus, and shales, seemingly of the same 

 series, yield Dicranograptus at one outcrop, and Dicellograptus at 

 another. 



(3) The brachiopods, corals and crinoids obtained from the 

 Grits present a mid or newer Silurian facies, with an admix- 

 ture of older forms. 



(4) The Grits seem to overlie normal Dicellograptus-Dicrano- 

 graptus shales, but have not been seen interstratified with such 

 shales. 



(5) The Grits underlie the Kerrie "Conglomerate north-west 

 of Riddell. 



Our conclusions are as follow : — 



(i.) The Grits, with their associated shales, etc., represent a 

 shallower water series than the normal Upper Ordovician grapto- 

 lite shales. 



(ii.) They are probably of Upper Ordovician age, and since 

 they overlie Dicranograptus beds, are probably late Upper Ordo- 

 vician. 



(iii.) From (a) their resemblance in texture, etc., to that of 

 Silurian rocks as at Springfield, and (b) the absence of unmis- 

 takably overlying Upper Ordovician rocks we think it probable 

 that the Riddell Grits were formed towards the close of the 

 Upper Ordovician, probably during differential movements, 

 which closed the Ordovician, and ushered" in the Silurian. In 

 placing the Grits as Upper Ordovician, we emphasise their grap- 

 tolite fauna more than the brachiopods and other forms. Diplo- 

 graptus and Climacograptus range into the Lower Silurian, but 

 Glossograptus, Dicranograptus and Dicellograptus do not. Still, 

 the only graptolites found in undoubted Grits have been Diplo- 

 graptus and Climacograptus. 



