DOLERITES AND EHYOLITES OF NATAL AND ZULULAND. 153 



A typical specimen of a pitchstone-like rock (116) comes 

 from Manual! Creek near Crossly's Store where the rhyolites 

 are in contact with Cretaceous beds. This rock shows under 

 the microscope fairly numerous but small phenocrysts of 

 unstriated felspar, with a few small and altered pale green 

 augites, in a glass showing well-marked flow-structure and 

 less pronounced perlitic structure, and rendered dense by 

 thickly distributed short needle-like microliths. The felspar 

 phenocrysts, although showing no twin-striations, are pro- 

 bably near to anorthoclase as the refraction is about equal to 

 that of Canada-balsam. An interesting accessory constituent, 

 occurring in one or two small crystals, is in all probability 

 allanite : one of the sections is a twin with symmetrical 

 extinction of about 32°, and another shows oblique extinction 

 of about 38° and pleochroism from dark brown (nearly 

 opaque) to reddish-brown : traces of cerium earths were 

 detected in the course of the chemical analysis of the rock 

 (see p. 154). 



Other specimens from Crossly's Store show similar pheno- 

 crysts of orthoclase and anorthoclase in a base which is no 

 longer glassy but devitrified and incipiently spherulitic. 

 Still other specimens (408) are brecciated, and consist mainly 

 of fragments of a beautifully perlitic glass, banded brown and 

 white,. In a rhyolitic agglomerate (112) from the same 

 locality some of the fragments consist of a mass of minute 

 spherulites giving well-marked crosses between crossed 

 nicols. 



A chemical analysis of the specimen (116) from Manuan 

 Creek described above gave the following result under I. 



The numbers under II were obtained in the analysis of 

 another sample of the glass, which was made in order to 

 confirm the comparatively high percentage of alumina. Under 



III is given the molecular composition of the rock, and under 



IV the analysis of a spherulitic " liparite pitchstone " from 

 Silver Cliff, Colorado.^ 



» Eakiiis and Cross. ' Bull. Phil. Soc. Wash..' xi, 1891, p. 420. 



