DETAILED DESCRIPTIONS OF ROCKS. 49 



pressure, from the sides of a great fissure through which the rock was 

 erupted, and that this may be the site of a vent. The fissile rock 

 extends over an area of about six miles in its longer diameter from 

 N. W. to S. E., but it may extend to a greater distance beneath the 

 sand and alluvium beyond the edge of the hills ; the width seems to be 

 very irregular, from two to three miles. 



At the south-west extremity of this group of hills a thick lava 

 flow shows a strongly developed columnar structure, with lines of 

 flow running transversely to the columns. A section across the end of 

 a spur formed by this flow shows that the columns are arranged in a 

 roughly radial fashion, lying nearly horizontally towards the side of the 

 spur and inclined at various angles towards the centre, like the radial 

 structure so often seen in dykes of basic rock. Some of the columns 

 are traversed by joints showing a cup and ball structure. 



In the dry watercourses at the western edge of these hills a 

 breccia occurs formed of clips and fragments of glass imbedded in a 

 glassy matrix. The process of devitrification in the imbedded frag- 

 ments is very well exhibited (PI. XI, figs. 4, 5). This bed can be traced 

 for about three miles along the edge of the hills. 



A band of sphaerulitic rhyolite, in which the sphaerulites are some- 

 times as large as hazel nuts, is exposed in a ridge running west from 

 the hill with a survey mark about three miles south-west of Nagona 

 and can be traced for some distance along the side of the ridge between 

 two flows of the porphyritic rhyolite. A beautiful perlitic structure 

 is also developed in portions of the rock (PI. X, fig. 6) ; and the same 

 structure occurs in a flow forming a ridge running south from the 

 village of Madli, about two miles to the north of Nagona. 



About eight miles to the south-west of Nagona another group of 

 isolated hills occurs extending from the village of Tesingri (Tringri of 

 map) in a westerly direction for about 10 miles to near Patodi. Those 

 at Tesingri consist of compact dark coloured rhyolites, with well devel- 

 oped flow-structure and columnar jointing, with a general dip at low 



( 49 ) 



