THE METAMORPHIC ROCKS OF ADELIE LAND. STILLWELL. 37 



colour as in Nos. 720 and 635. Lawsonite appears in the same manner as in No. 720. 

 The iron ore consists of magnetite and pyrite with some reddish hematite. The rock 

 may be called a lawsonite amphibolite schist. 



No. 635. This example is a typical one with the parallel arrangement of hornblende 

 crystals. The hornblende is the most abundant mineral and is developed in long prisms. 

 Like No. 634A, the felspar is in strong contrast to No. 634, where it is mostly clear, 

 while here it is nearly all saussuritised. Occasional clear fragments arise among the 

 saussurite products. Grains of quartz, epidote, and mica can be distinguished in the 

 decomposed mass. The biotite is not abundant and contributes little to the schistosity 

 of the rock. Its colour is not quite normal. Basal sections possess a biscuit brown 

 colour, and cross sections are pleochroic from reddish brown to colourless. Consequently 

 the biotite possesses a curious bleached appearance. Lawsonite is well developed and 

 reaches 3 per cent., and was identified in this section before the preparation of the thin 

 sections of Nos. 720 and 634A. This colourless mineral is frequently found in parallel 

 growth with the biotite (Plate I., fig. 4). Sometimes it is so developed after this manner 

 that the biotite appears to be merely threaded in along its cleavage planes. Its form 

 is usually lobated, and the biotite plates, in consequence, bend around its contour. 

 Its cleavage is well developed and parallel to the elongation of the crystal and the cleavage 

 of the biotite. Apart from its association with biotite, it may be found in grains among 

 the saussuritised masses, and it also appears in small microscopic segregation veins. 

 The outline of the sections is frequently granular, but in the veins it may be rectangular, 

 and these show second order polarisation colours. Some sections, with low polarisation 

 colours, have a distinct tendency to a rhombic outline. Wherever the cleavage or 

 crystalline form is observable the mineral is found to have straight extinction. Further, 

 its biaxial character can be verified, and hence it is orthorhombic. Its optical character 

 is positive. Refractive index is high, higher than biotite and hornblende with which 

 it appears in contact. The birefringence is high, and second and third order colours 

 are seen. Some sections, however, possess quite low first order colours, indicating 

 that two of the three principal refractive indices are not much different. These 

 characters have fixed the identification as lawsonite. The lawsonite is not unlike a 

 colourless epidote in appearance, but the latter would not have uniform straight 

 extinction. Besides, epidote is present in isolated pleochroic grains of pale yellow 

 colour and may be compared with lawsonite in the same field of view. The iron ore 

 consists of scattered grains of magnetite and pyrite. Sphene is practically absent, 

 and apatite is found in occasional needles. No. 635 is a lawsonite amphibolite schist. 



Summary of Microscopical Characters. 



The dominating constituent throughout the series is the dark-coloured ferromag- 

 nesian mineral, either biotite or hornblende, or both. The biotite is most important in 

 the mica schists and the hornblende is most important in the amphibolites. The biotite 

 is fresh and clear, and the Z colour is normally brown, varying to a reddish brown. 

 In exceptional cases it is green, and both types may be found in the one section. Indeed, 



