182 PETROLOGICAL NOTES ON SOME OF THE ERRATICS 



Corundum in the form of sapphire is quite a notable constituent. It occurs in 

 tabular crystals, O'l mm. by - 02 mm., very much corroded by the magma. It is not 

 noticeably pleochroic. It is included in all the other minerals except apatite, of which 

 there is a little. 



There are irregular glassy veins about *05 mm. in width running through the rock. 

 Sanidine crystals project from the sides, suggesting a miarolitic fabric. Here and there 

 are tufts of fibrous radial mineral like that described in the inclusions in the felspar. 

 The interstices are filled with colourless glass (or possibly opal). 



444. Spherulitic Trachyte 



Macroscopic Characters. — Very fine-grained rock of brownish- grey colour, 

 with rather scanty phenocrysts of felspar up to 3 mm. by 1 mm., and still less 

 abundant thin flakes of biotite up to 5 mm. by 1 mm. 



Microscopic Character s. — Contains phenocrysts of felspar, biotite, and 

 magnetite in a finely spherulitic base. 



The base consists mostly of microlites of felspar and biotite with a little interstitial 

 quartz, and felspar of a character different from that of the microlites. This interstitial 

 felspar is certainly orthoclase. 



The microlites have rather a rosette grouping than a spherulitic arrangement. 

 Many of the microlites are distinctly cored. 



The main part of the crystal extinguishes at 5|° from the length, while the core 

 extinguishes at 22°. The highest extinctions measured in such microlites is 12°, and 

 the elongation is negative. 



Quartz and biotite are not abundant; the former shows very fine granophyric 

 structure in places. 



The phenocrysts of felspar are most remarkable in exhibiting a veritable " bearded " 

 structure. The main crystals are strongly idiomorphic and apparently consist of anortho- 

 clase and plagioclase. Rather unsatisfactory measurements indicate a normal andesine 

 for the latter, which show complicated twinning after pericline, albite, Carlsbad, and 

 Manebach laws. 



While any of the porphyritic felspars may show the remarkable outgrowths above 

 mentioned they are much more frequent and striking on the anorthoclase. 



Along the direction of the brachy- diagonal the microlites are closely set and parallel 

 to the crystal axis, but on other faces and on fracture they tend to arrange them- 

 selves in rosettes. In character they are similar to the microlites of the base except 

 that none were noticed showing the forked structure described above. The extinction 

 angle is 14° from the length of the fibres. 



Biotite phenocrysts are not very abundant and are mostly associated with pheno- 

 crysts of felspar or included in masses of the latter. This rather suggests that some 

 of the larger felspar crystals, together with these aggregates, may be included fragments 

 of older rocks. The biotite shows light yellow to very dark reddish-brown pleochroic 

 colours, and is crowded with apatite crystals and contains also a few minute zircons 

 with pleochroic halos. Side by side with perfectly fresh crystals of biotite occur others 

 of similar size and habit completely converted into bright green penninite. 



433. Dense Porphyritic Basalt 



Microscopic Characters. — The base is a very dense aggregate of minute 

 brown needles of augite and tiny octahedra of magnetite with somewhat larger 

 microlites of plagioclase whose composition is about Ab 3 An 4 . 



