Sedimentary Rocks. 59* 



every outcrop of sandstone and grit discovered. It was found 

 that rocks outcropping at numerous places in the district could 

 conveniently be grouped together, and for convenience we have 

 called the series the Riddell Grits. The undoubted Upper Ordo- 

 vician rocks have already been described — homogeneous sand- 

 stones and carbonaceous shales, usually finely laminated, and in 

 extreme cases weathering to a soft clay. The most prominent 

 beds of the Riddell Grits are of coarse sandstone and grit; in 

 some places, as in Jackson's Creek, above " Dalrymples','' a gravel 

 conglomerate. The bands are persistent, but within any band 

 the texture changes, passing from grit to a fine sandstone or 

 quartzite, evidently the effect of currents during deposition. The 

 grit bands are slickensided, and their outer layers often show 

 bulges and channels where they have been forced into less re- 

 sistant rocks. In the coarser grits there are smooth-lined cavities 

 resembling casts of fossils, but most likely caused by the removal 

 by solution of small clayey patches. The grits are fossiliferous, 

 but such is the nature of the rock, that well preserved fossils 

 are unobtainable. Brachiopod casts, occasional gasteropods,. 

 corals, polyzoa, and most commonly the impressions of crinoid 

 stems, are to be found. Mr. Chapman very kindly examined a 

 large quantity of unsatisfactory material for us, and his iden- 

 tifications are given on pages* 69-71. 



With the grits, and in some localities, interstratified with char- 

 acteristic grit bands, are mudstones or shales, brown, rubbly, and 

 less finely laminated than the typical graptolite shales. There 

 are also thin-bedded fine micaceous sandstones or arenaceous 

 shales. It will be seen that the Riddell Grits represent a more 

 shallow water deposit than the Upper Ordovician rocks already 

 deak with. Determination of their age is fraught with difficulty. 

 The following points have to be considered : — ■ 



(i.) It is impossible to draw a sharp distinction between 

 shales associated with the Grits, and normal Upper 

 Ordovician shales, and the relation of the Riddell' 

 Grits to the Upper Ordovician may depend on the 

 relation of the shales of the two series. As in the 

 case of Lower and Upper Ordovician, typical shales- 

 of the two series differ greatly from each other, but 

 no safe test can be applied to a limited outcrop. On- 

 account of this difficulty, and because of the nature 

 of the country, it is difficult to ascertain the field re- 

 lations of the Grits. 



