Speight. — On a Dolcritic Dyke at Dyers Pass. 409 



nearly vertical. There are other outcrops up the western side 

 of the gully, and at a road on the other side of the spur (d), and 

 again on an adjoining spur (e), where it has been quarried. 

 The country rock is an olivine-andesite, but in the neighbour- 

 hood beds of laterite occur, produced by the weathering of 

 the andesitic and basaltic flows, and by the subsequent pouring 

 over them of hot lava streams. In no place were the beds 

 observed to overlie the dyke, so it is most probably of later 

 origin. The country rock has been altered near the edge 

 of the dyke into a black rock in which crystals of feldspar 

 appear, while a little further away it has become a laterite. 



The dyke is composed of a dark-grey rock on fresh ex- 

 posure, but near the edge it becomes black. It is more crystal- 

 line in the middle, but contains vesicles throughout. These 

 are not filled with infiltration products. Near the edge occurs 

 a band of steam-holes, with their long diameters parallel to 

 the edge of the dyke. There is not any well-marked columnar 

 ■structure such as occurs in the dyke above. 



Specific Gravity. 

 By weighing pieces of the rock in water a specific gravity 

 of 2-77 was obtained, but on grinding to a fine powder, and 

 using a specific-gravity bottle, a result of 2-86 was obtained. 

 This is what we should expect from the vesicular nature of 

 the rock. The further examination of the rock was carried 

 on — (1) By a quantitative chemical analysis ; (2) by a mi- 

 croscopical determination by means of thin sections. 



I. Chemical Analysis. 

 The following result was obtained : — 

 Loss on ignition 

 SiO, 

 AlA 

 Fe-A 

 Feb 

 CaO 

 MgO 

 K,0 

 Na^O 



100-36 

 This corresponds fairly closely with a dolerite of Hailstone 

 Hill, Kowley, with the exception of the absence of TiOa. 

 (Teall's " British Petrography," p. 213.) It is well within the 

 basic series, as is shown by the percentage of SiOj. The 

 large proportion of CaO may be accounted for by the presence 

 of anorthite as the porphyritic feldspar. This was never large 

 enough to be analysed separately. 



