Rocks near Heathcote. 329 



on Red Hill, and here the alteration effected by the granitic in- 

 trusion is cei'tainly trivial, amounting' only to a slight bleaching 

 in coloiu*, and the production of a rather fissilei-jointed character 

 in the diabase for a distance of about three feet from the con- 

 tact. The labradorite^porphyrite occurs as a small intrusion in 

 the diabase, just north of Photograph Knob, and as a marginal 

 ruck at the junction of the micro-granite and diabase a.t Red 

 Hill. It is a dark rock (No. 599) with porphyritic labradorite, 

 biotite and hornblende, in a dense granophjTic groundmass of 

 quartz and orthoclase. It must be regarded as a modification 

 of the micro-granite which may possibly have absorbed some 

 diabasio material before consiolidation. 



8.— Nature a\d Origix of the Ordovician Sediment?!. 



This question has been discussed in considering the relation 

 of the Ordovician and cherty series to the diabases, and the 

 appearances of Ordovician shales in hand specimens, and under 

 the microscope have been compared with those of fine-grained 

 diabase rocks. It has been sho^vn that at the S. end of Red 

 Hill a continuous conformable succession is seen from a rock 

 composed of diabase fragments, through finer-graine-d types to 

 the normal Ordovician shales. The evidence points strongly to 

 the Ordovician series, near the diabase, being composed malnlv 

 of diabase fragments. This is supported by the observation, first 

 made by Mr. Dunn, which can be verified at several localities, 

 ihat raagnesite nodules segi"egate from the Ordovician shales. 

 The junction between the foliated diabase of Red Hill and the 

 d'abasic Ordovician is a sharp one (PI. XVII., Fig. 2). It may be 

 explained either as an unconformable junction or as a fault junr- 

 tiiip. In the first case the diabase might be pre-Ordnvician, in 

 the latter the diabase may be of Lower Ordovician age. and con- 

 sist of unbedded tuffs passing into a bedded series. 



The Dinesus beds, which also are mainly composed of diabase 

 fragments, appear to be interbedded submarine tuffs ; they may 

 ])ossibly be, however, detrital sediments formed from a pre- 

 Ordovician diabase. No section showing the relations of the tw.i 

 rocks has been observed. 



