Igneous Fehhles. 291 



tion, and therefore, inferences drawn from the results of a micro- 

 scopic examination of thin sections of these rocks, must be accepted 

 with caution. Associated interbedded lavas or tuffs are absent, 

 and the igneous fragments present in the limestone can be most 

 easily explained on the assumption that they were derived from 

 the same source as the detrital igneous pebbles. No indubitable 

 Silurian igneous rocks have been recognised in Victoria, and moro 

 definite evidence than that brought forward by Mr. Chapman, is 

 needed to establish Silurian vulcanicity. The present writer agrees 

 with Mr. Herman that the conglomerates and grits are normal 

 ehore line deposits, and that the included igneous pebbles are de- 

 rived from pre-existing formations. 



Petrology. 



Honihlende yiieiss. 



Specimen Wl, pebble in rduglomei-ate, section on road to Jubilee 

 Mine. 



The hand specimen of the rock is coarse grained. Felspar and 

 hornblende can be recognised megascopically. 



Examined microscopically, the rock shows very well the effects 

 of mechanical dynamic metamorphism. Strain polarisation has 

 been developed in the quartz grains, and they are considerably 

 mylonitised. The horneblende is almost entirely light green actino- 

 lite. The prisms and plates of this mineral are very rarely idio- 

 morphic, and are often twisted and bent in all directions. ■ The 

 elongation of the hornblende crystals is parallel to the direction 

 of foliation of the rock. They are occasionally twinned according 

 to the usual law; w4th twin and composition plane 100. One 

 idiomorphic basal section showed an excentric emergence of an 

 optic axis. Determination of the axial plane showed that it 

 bisected the obtuse angle between the cleavages, and that it was 

 parallel to the clinopinacoid, as is usual for hornblende. It is 

 moderately pleochroic, giving the following colour scheme : — 

 X light yellow green. 

 Y light green. 

 Z bright green, and Z>Y>X as usual. 



The maximum extinction angle from the prismatic cleavage is 15o. 

 The felspar is greatly kaolinised, and has suffered a considei-able 

 amount of mechanical deformation. Plagioclase appears to be 

 in excess of orthoclase. The refractive index of the plagioclase is 



