﻿part 2] 



THE PTCRITE-TESCHEMTE SILL OP LIT GAR. 



97 



other constituents but analcite, and minute uniformly- scattered cubes of 

 magnetite, all embedded in a limpid, colourless, isotropic base which has the 

 cleavage and low refraction of analcite. The latter is occasionally segregated 

 into small areas comparatively free from the other constituents. A few small 

 patches of serpentinized material occur, which may represent ohvine. The 

 extinction of the felspar is between 20° and 30°, but the fine grain forbids 

 exact measurement ; it is probably to be referred to acid labradorite. The 

 following is a rough estimate of the mineral percentage : — plagioclase 

 '(Ab T An 1 ) 45, augite 40, biotite 5, magnetite 5, analcite 5. 



(b) Adjacent to the above, with a sharp boundary between the two, is a 

 band composed of the same minerals, but distinctly finer in grain. The 

 proportion of augite has increased at the expense of plagioclase and analcite. 



(c) The next band is still denser. The minerals are the same as above, 

 but are so crowded together as practically to exclude the isotropic base. 

 Magnetite is more abundant, and is sprinkled very uniformly over the field. 



(d) A coarse vein of teschenitic material separates (c) from (d), which is 

 the densest of all the bands occurring in the slides. It consists of a crowded 

 mass of minute microlites of augite and plagioclase, with very numerous, 

 small, irregular, poikilitic plates of biotite, all dusted over with magnetite. 

 There are sparse microphenocrysts of plagioclase and purple augite. The 

 felspar has dwindled, and' the rock approximates in composition to the 

 monchiquites. The colourless isotropic base is still to be recognized with a 

 powerful objective, and can be definitely identified as analcite, where, in 

 places, it is comparatively free from augite and magnetite. 



Apart from the coarse teschenitic veins the four schlieren 

 described above are the principal textural varieties to be found in 

 the slide. But within each of them occur slighter variations in 

 texture and composition, and no fewer than eight distinct varieties 

 of rock occur within the limits of a thin section half an inch long. 



These rocks are basaltic in aspect, and in accordance with their 

 occurrence as the contact-facies of a teschenite, they niay be called 

 teschenite-basalts. The term analcite-basalt is unsuitable, 

 as it has been used for definite lava-form rocks. Moreover, this 

 rock is practically devoid of olivine, which is abundant in the great 

 majority of analcite-basalts. Some of the schlieren approximate 

 to monchiquites, and others to biotite-basalts. 



The contact-facies gradually merges into teschenite by increasing 

 granularity and enlarged proportion of analcite. The following will 

 serve to give an idea of the intermediate fine-grained teschenitic 

 rock. The thin section is from a specimen obtained immediately 

 above the lower contact-facies in the Grlenmuir Water. Micro- 

 scopically the rock is somewhat similar to the coarser schlieren of 

 the contact-facies, but is not banded and is not so rich in ferro- 

 magnesian minerals. It consists of numerous, small, euhedral to 

 subhedral prisms of pale augite, with stout plagioclase-laths, some 

 serpentinized olivine, and leucoxenic ilmenite, in a ground-mass of 

 dusty analcite. Associated with the ilmenite are numerous minute 

 scraps of biotite. A few turbid areas have the shape, low double 

 refraction, and general aspect of nepheline as it is more recogniz- 

 ably developed in rocks to be described hereafter. Apatite needles 

 are abundant in the areas of analcite. The rock may be described 

 as a fine-grained teschenite; or if it be desired to emphasize its 

 origin as a contact-facies it may be called teschenite-dolerite. 



