Rocks near Heatkcote 317 



Silurian, Pliocene and Alluvial deposits I have accepted without 

 conlirmation, as the problems discussed in this paper are not 

 naaterially concerned with these rocks. The boimdaries of the 

 igneous rocRg and of the diabase Ordovician junction I have 

 examined fairly closely, and while not claiming that the revised 

 boiiiKhiries I have laid down are absolutely accurate in every 

 detail, 1 feel confident they represent a sufficiently close approxi- 

 mation to the true relations for the purposes of a sketch map and 

 for this discussion. In that part of the map dealing with the 

 junction.s of diabase and Ordovician 1 have made some i)npor- 

 tant modifications. The marginal silicification of the Ordovi- 

 cians is represented by dotted lines. The interesting relations 

 near Photograph Knob have been closely examined (see Sketch 

 Section), and I have come to the conclusion that the bay of 

 unaltered Ordovician represented on the map in Mr. Hewitt's 

 report just north of the felspar porphyrite, is in reality diabase 

 tuff and silicified diabase approaching a chert, while the 

 patch marked nietamorphic has no defined boundaries, and is 

 really highly silicified diabase and diabase tuff. Probably a 

 dense bluish grey rock which is represented here was accepted by 

 the author of the map, and apparently by Prof. Gregory, as a 

 blue Ordovician shale. It shows, however, no sign of bedding, 

 and in section under the microscope (Sect. 614) differs from the 

 shales (Sect. 574) by showing an absence of bedding and of 

 micaceous flakes. It appears to consist almost wholly of chloritic 

 or serpontinous alterations of a basic rock, probably a tuff. A 

 foliated bluish grey rock of somerwhat similar character is seen 

 in section in a small gully immediately south of Photograph 

 Knob, and a similar rock (Sect. 597), splitting readily into 

 long semi-prismatic masses, occurs in paddock 17c, about a 

 quarter of a mile S.E. of Photograph Knob, and within 

 the boundary mapped as diabase. The same rock has then 

 in one place been mapped as unaltered Ordovician, and a 

 little further to the S.E. as diabase. There can be little doubt 

 that both outcrops are related to the igneous series, and prob- 

 ably represent fine-grained consulidated diabase tuffs. This is 

 supported by the fact that at the S. end of Red Hill, in the shaly 

 diabase, occurs an indurated rock (Sect. 633), agi-eeing closely in 

 hand specimen and under the microscope with the rock N. of 



