408 Transactions. — Geology. 



Akt. XLYII. — On a Doleritic Dyke at Dyers Pass. 



By E. Speight, M.A. 



[Read before the Philosophical Institute of Canterbury, 4th October, 1893.] 



Plates XLIX.-L. 



Locality. 

 On the Dyer's Pass Eoad, over the Port Hills to Governor's 

 Bay, there occur several dykes. One of them (PL L., a), just 

 above the Convalescent Home, runs north-west and south-east. 

 It has been quarried for some distance, and is about 6ft. broad, 

 and nearly vertical. It is probably classified as an andesite, 

 but the deep weathering has rendered an accurate determina- 

 tion difficult. It seems to be composed of an interlacing 

 network of feldspar microliths and augite grains, in which are 

 porphyritic crystals of feldspar. This is probably labradorite, 

 since the extinction-angles between twin lamella; are greater 

 than 37°, but not greater than 63° ; also, a crystal in a sec- 

 tion which shows no twinning, and is therefore probably 

 parallel to the brachypinacoid, gave an optic axis out of 

 the field, and a revolving axial shadow. There is no other 

 porphyritic mineral. Probably augite is present, but only one 

 large crystal was noticed, and that was greatly altered. The 

 feldspar of the ground-mass is probably oligoclase, as the ex- 

 tinction is nearly parallel to the length of the microliths, and 

 never greater than 5° ; but the determination from the extinc- 

 tion of twin lamella3 did not give satisfactory results on ac- 

 count of undulose extinction. The augite of the ground-mass 

 is greatly altered, forming brown grains of limonite. 



Higher up on the road, on the south side, there is another 

 dyke (b), running also north-west and south-east. It has not 

 been quarried, and so it was very difficult to get satisfactory 

 sections. It is about 10ft. wide, and shows a rudely columnar 

 structure perpendicular to the sides of the dyke. The ends 

 of the columns are easily seen, as on the lower side it appears 

 as a wall about 12ft. high. This is probably basaltic, as 

 sections show olivine to be present, though in all I obtained 

 it is altered to limonite. 



Below this dyke occurs an old quarry in a corner of a small 

 gully (c). This has not been worked for some time, but quanti- 

 ties of stone lie about. This has been derived from a large 

 dyke about 15ft. broad. The dyke runs generally in a north- 

 westerly direction, but where the quarry occurs it has been 

 bent, so that the two lines make an angle of about 15°. The 

 dip of the eastern branch is about 75°, while the western is 



