DOLERITES AND IIHYOLITES OF NATAL AND ZULULAND. 145 



dules of calcite and epidote or green earth, they show for the 

 most part a granular intersertal or variolitic structure and other 

 characters similar to those exhibited by contact-specimens 

 of undoubtedly intrusive rocks, such as the dyke-rock (73) 

 cutting black shales three miles east of Incongwane Hill, 

 Zululand, the dolerite (177) in sandstone near Glendale, and 

 the sill (191) in coal-measure sandstone north of Isebeni 

 Creek, between the mouths of the Umhlali and Umvoti 

 Rivers, Natal. In connection with these basaltic rocks, a 

 specimen (179) from three miles north of the mouth of the 

 Tongaat River, Natal, which shows an absolutely abrupt 

 passage from a coarse-grained dolerite to a fine-grained 

 basalt, is noteworthy. The dolerite of this specimen consists 

 of a coarse-grained aggregate of plates of labradorite, pris- 

 matic colourless augites, and much interstitial micro-pegmatitic 

 material, while the basalt in contact Avith it shows small 

 porphyritic labradorites, colourless augites, and a few altered 

 olivines (some surrounded by a fringe of purplish augite) in a 

 base dense with magnetite. 



Of more compact and finei'-grained structure than these 

 basaltic rocks are other specimens which were doubtless taken 

 close to the contact with the sandstone or shale, into which 

 the dolerites were intruded, for they present a similar grada- 

 tion in structural characters to that exhibited by dolerites of 

 the Ferrar Glacier, South Victoria Land, taken within six 

 inches of the contact with the sandstone.^ Very similar to 

 the rock figured in PI. X, fig. 3 of the ' Discovery Report,' 

 are, for example, specimens from a dolerite (74) intrusive in 

 black shales in the bed of the Umsunduzi River, two miles 

 east of Incongwane Hill, Zululand, and from a dyke (24) 17 

 feet wide at the 200 feet workings, Newcastle Colliery, Natal. 

 These consist of radiating felspar laths and small prismatic 

 augites, with interstitial patches of base black Avith magnetite. 

 Other specimens show a radiating variolitic structure like 

 that figured in PI. X, fig. 4 of the ' Discovery Report.' One 

 such specimen (205) from a sill intrusive in shales on Umhlali 

 ' See i.e., vol. i, 'Geology.' pi. x. figs. 2-5. 



