278 N. R Jaiimn-: 



sericite, and a very little ilmenite. The texture is i^orphyritic. 

 Flow banding is very well developed, yet the ground mass is 

 thoroughly crystalline. 



A beautiful blue tourmaline occurs scattered through the section 

 in mossy aggregates. These aggregates examined under the high- 

 power resolve into groups of acicular crystals and hexagonal cross 

 sections of these needles. Pleochroism is very marked, varying from 

 ultramarine to yellowish or greenish-brown. An anomalous feature 

 of this mineral is that its strongest absorption is in the same 

 direction as in biotite, which also occurs in the same section. It 

 seems probable that the tourmaline replaces biotite and that it has 

 retained the original form of the mica. 



Minute flakes and prismatic sections of biotite occur throughout 

 the section. These are probably secondary in origin. The original 

 biotite differs from them in having much larger sections, and in its 

 corrosion by the magma and separation of oxides of iron and 

 titanium. A brown, isotropic mineral, having a refractive index 

 Isfia than the balsam and occurring in small amount in the section, 

 is opal. It is fringed with secondary biotite in places. 



Section No. H57, Acheron River, above St. Fillans. — This rock 

 consists of phenocrysts of deeply embayed colourless quartz, turbid 

 microperthitic orthoclase, and a little biotite and albite, in a 

 cryptocrystalline ground mass which was originally glassy and 

 microspherulitic. Minute spherulites, preserved in quartz, are not 

 uncommon in the ground mass of the rock. Chlorite and brown 

 iron oxide replace the original femic mineral which was probably 

 biotite. Prisms of tourmaline, showing normal absorption, are 

 associated witli chlorite or serpentine in places. 



Section No. H49, Acheron River, above St. Fillans.- — Aggregates 

 of blue tourmaline showing anomalous absorption are present in 

 this section. The tourmaline occurs associated with muscovite and 

 replaces biotite, whose form it retains. Plagiocluse phenocrysts are 

 absent, but a little albite occurs in the ground mass of the rock. 



Section No. H67, banded rhyolite from near Malory's falls. — 

 Chalcedonic silica, with nuclei of slightly yellow coloured, appa- 

 rently isotropic material, having a refractive index nmch greater 

 than the chalcedony, occurs in places in this section. 



Certain obscure flinty rocks occurring near Mount St. Leonard 

 may be best described under the heading of rhyolites, although the 

 writer is quite prepared to admit the possibility of tlieir being 

 silicified tuffs. 



