Cherts and Diah<ii<e Rocl-x. 243 



stones, we come to more iudm-ated shales, aiid moving on to- 

 wards the metamorphic area the shales show more and more 

 silicitication, until they ultimately pass into undoubted cherts. 

 The sandstones do not show the same amount of alteration, but 

 become more micaceous. It must be stated that the evidence 

 in this area is not as satisfactory as one could wish, because 

 the surface is covered with soil, and the nature of the under- 

 lying rock can only be judged by the fragmentary material on 

 the surface. 



About tlii-ee miles north of Tatong township a triangular 

 area of silurian rocks is shown in Mr. Hewitt's map. The rocks 

 in this area consist of sandstones, quartzite, slates, together with 

 bands of what may be termed chertified slates. These beds are 

 intensely indurated, but the silicitication is not quite as far 

 advanced as in the case of the typical chex'ts, and I think they 

 undoubtedly represent an intermediate stage between the cherts 

 and normal slates. 



Relation of the Rocks in the Area. 



In the area under consideration tlie following palaeozoic rocks 

 have been recorded: — Heathcotian (1), Upper Ordovician, Sil- 

 urian, Lower Devonian porphyries and Upper Devonian conglo- 

 merates, while to the south-east are the Carboniferous sand- 

 stones of Mansfield. 



The age ascribed tii the porph}Ties seems connect, as they are 

 intrusive into the Silurian in the Broken River valley, and are 

 overlain to the south-east by the Lower Carboniferous sand- 

 stones. Flanking the porphjTies. and apparently resting on 

 them, are beds of conglomerate, which the Survey have recorded 

 aa of Upper Devonian age. These conglomerates consist of 

 rounded pebbles of quartzite, with occasional fragments of chert, 

 and where the conglomerate occurs near the diabase, numerous 

 diabasic pebbles are found. There is an entire absence of 

 porphyr)' pebbles in the conglomerate, even where the conglo- 

 merate is directly in contact with the porphyiy. In the bed of 

 the Holland's, about half a mile below Dodd's Crossing, the 

 porphyry is clearly seen intrusive into the conglomerate, so 

 that the conglomerate beds ai-e older than the porphyiy. and 

 therefore are probably of Silurian age. If fux'ther evidence were 



