316 Ernest W. iSkeats : 



crosses a small creek by a bridge within 20 yards of the Murray 

 road. The Silurian is mapped as being "in situ" on the east 

 side of tiie Murray road, and beneath the bridge in the bed of 

 the oreek are large blocks of Silurian quartzite and conglomerate 

 not " in situ," but evidently derived from rocks near at hand. 

 These blocks in several instances contained pebbles and angular 

 fragments of black cherts. Microscopic examination of these 

 chert fragments shows that they are petrologically indistinguish- 

 able from the black cherts of Lady's Pass, and near Red Hill. 

 This corroborates Lidgey's evidence, and shows Professor Gregory 

 was right in claiming the cherts, and presumably the diabase, as 

 of pre-Silurian age. 



5. — Rblations of the Igneous Rocks and Black Cherts to thb 

 Ordovician Series. 



This problem is the most obscure of all those which arise in the 

 Heathcote district, and, as the previous literature shows, Pro- 

 fessor Gregory and Mr. Howitt have given explanations of the 

 relations of the two series which are mutually oonflioting. Mr. 

 Howitt claimed that the diabase has everywhere altered the 

 Ordovician rocks for some distance from the contact, the nature 

 of the alteration in places being bleaching and the formation of 

 secondary micas in the shales, and in other cases the silicification 

 of the shales ultimately to black cherts. Professor Gregory has 

 not dealt with this aspect of the question except to state that in 

 several localities, siome of which he cited, the Ord >vician at the 

 contact with the diabase was quite unaltered. 



I have followed the line of contact of these two series from 

 Photograph Knob in the S.E., almost continuously to three miles 

 north of the seh\'ynite outcrop in the northern part of the area, 

 and without at this stage discussing the origin of the alteration 

 wherever I have examined them. I have found the Ordovician 

 ro'cks silicified near the junction with the diabase to a greater or 

 lesser extent. On the accompanying sketch Geological map 

 (PI. XATII.) T have indicated this by a series of dots added to the 

 horizontal shading, which represents the Ordovician rocks. The 

 map is based upon those published in Mr. Howitt's report by 

 Messrs. Lidgev and 0. A. L. Whitelaw. The boundaries of the 



