210 MR. H. H. THOMAS ON THE [M"ay I907, 



The vesicles are usually rilled by zeolites, but sometimes by secondary 

 quartz and also chlorite. 



Some of the flows are much decomposed, and occasional specimens, 

 such as F 1037, show a mass of felted felspar-microliths silicified 

 and highly iron-stained. 



Unusual Types. 



F 1034. Locality: Near the fault, Deka Eiver, near the railway-bridge. 



This rock in the hand-specimen shows large porphyritic felspars 

 set in a red, highly ferruginous ground-mass, which is apparently 

 wholly crystalline. Under the microscope the porphyritic felspars 

 are seen to be twinned according to the Carlsbad and albite-laws, 

 and to belong to a species of labradorite. The groundmass con- 

 tains olivine now converted into yellow pseudomorphs, sometimes 

 showing the original crystal-outline. Augite occurs in small, mostly 

 idiomorphic crystals, and also plentifully in the groundmass. The 

 felspar-microliths are labradorite : they are much twinned (Carlsbad 

 and albite-laws) and lack any definite arrangement. There is a 

 good deal of interstitial material which was originally glass, now 

 rendered absolutely opaque by limonitic iron-ores. 



The rock, as a whole, is slightly vesicular ; the vesicles, which 

 range up to half an inch in diameter, are filled with stilbite and 

 lined with chlorite. Locally it shows signs of brecciation, and from 

 this and other properties it is suggested that it is the margin of an 

 intrusion or flow. , 



F 1031. Locality: 2 miles south of 'Klaas's, near the head of the 

 Matetsi. 



A remarkably fresh-looking, dark greyish-brown rock, in which 

 light amber-coloured grains and crystals are very noticeable. 

 Under the microscope it is seen to ally itself to the subophitic or 

 glomeroporphyritic examples described above, but to differ in the 

 occurrence of an abundance of a bright-yellow mineral of secondary 

 origin. This mineral, which is fibrous and strongly refracting, 

 both replaces small crystals and grains of olivine and fills minute 

 cavities. 



In all other respects this rock may be grouped with the sub- 

 ophitic varieties already described. 



Fragmental Rocks, etc. 



F 1018. Locality : Eastern bank of Mavangu Creek, near Camp. 

 1023. Locality : South side of the Zambezi, floor of the gorge near 



the Namaruba confluence. 

 1027. Locality : Bwani River, north of the Zambezi. 



Among the fragmental rocks one true tuff was noticed, namely 

 F 1018. It consisted of broken felspars, almost opaque reddish 

 tachylytic glass, angular and vesicular masses of fine-grained 

 dolerite, set in a matrix of fine broken felspar-microliths and 

 clastic quartz. The other specimens are apparently flow-breccias : 

 they are largely made up of fragments of tachylytic glass as before* 



