Sedimentary Rocks. 75> 



Our conclusion is that the age of the Conglomerate depends 

 upon the age of the Riddell Grits, which, as we have already 

 stated pass under it, and in part at least, seem to have provided 

 the material of which it is formed. We have set out the con- 

 siderations which lead us to postulate a late Upper Ordovician 

 age for the Grits, and it seems to us probable that the Conglom- 

 erate may be a basal Lower Silurian deposit. This would not 

 be inconsistent with the enclosure of resorted material from it in 

 Lower Silurian mudstones as reported from Springfield. 38 (See 

 Figs. 2 and 3.) 



X. — Tertiary Gravels. 



Scattered throughout the district are deposits of river gravel. 

 In many cases they have been metamorphosed by the action of 

 the Newer Basalt flows which covered them, and show all grada- 

 tions from a comparatively loose gravel, such as t is found on the 

 gold-fields, to a homogeneous quartzite, in which all trace of 

 individual pebbles has been lost. The gradual change is well 

 shown near the junction of Riddell's and Jackson's Creeks. The 

 lowest gravels are hardly consolidated ; above them is a con- 

 glomerate, while just below the basalt the rock is quartzitic. It- 

 is possible that isolated outcrops of this rock have been mis- 

 taken for Ordovician, and this may account for the strip of bed 

 rock shown along Jackson's Creek on the Quarter Sheets, below 

 the mouth of Watson's Creek. As far as we can see, gravel alone 

 occurs in patches here. 



The highest gravels are on the flanks of Hare's Hill, and would 

 be about 1650 feet above sea level. Between this hill and Gis- 

 borne there are widespread sheets of gravel at levels from about 

 1350 to 1600 feet. Further south the levels are 1520 feet west 

 of Mount Gisborne, 1400-1500 feet around the Glendoon Spur,, 

 about 1300 feet at Breakneck Hill on the Melton Road, and 1000 

 feet a mile further south. While we were endeavouring to re- 

 construct the earlier contours by means of the alluvial gravels, 

 C. Fenner 39 had attacked the same problem by a consideration 

 of river grades. We have availed ourselves of his results which,, 

 like ours, indicate the existence of an east and west fault, called' 

 by him the Gisborne Fault. This name might, we think, be with: 

 advantage altered to " Toolern Fault," as the fault is several 

 miles south of Gisborne, while the fault which we have indi- 



38. 4, p. 42. 



39. 5, p. 266, 267. 



