582 



outlines. Much of the interstitial material is chloritic, and 

 in general the rock is considerably altered, but appears to 

 represent a rapidly-chilled rock, and may, therefore, be a 

 dyke. 



Slide No. 3. — Altered Gabbroid-Dolerite. 



This is a medium-grained, holocrystalline rock, with 

 abundant green ferromagnesian and lighter-coloured inter- 

 stitial material, somewhat altered, chiefly felspar. 



The thin section shows a holocrystalline and ophitio 

 structure. The ferromagnesian minerals are abundant, with 

 some prismatic outlines, but generally the secondary mole- 

 cular rearrangement has resulted in a confused, granular, and 

 fibrous intergrowth. Much of the original pyroxene has 

 passed over into uralite. Olivine is present, but not 

 abundant. The felspars are, generally, much altered, and in 

 habit are rather long prisms. From the extinction angles 

 they would appear to be> probably labradorite. 



2. Acid Porphyritic Rocks. 



These, in general, are pinkish-brown porphyritic rocks, 

 showing abundant phenocrysts of felspar and some quartz. 

 One occurrence of note shows this rock as intruding the 

 dolerite. There is some variation in the character and 

 appearance of this rock in the several outcrops noted ; some 

 of the extreme examples presenting rather nondescript fea- 

 tures as hand specimens. Thin sections, however, show that 

 this is due to the introduction of a considerable amount of 

 calcite by percolating solutions from the surrounding impure 

 limestones. This has led to a varying amount of metasomatic 

 replacement. The less altered examples, in thin sections, 

 show a well-developed porphyritic structure, in which the 

 prevailing phenocrysts are triclinic felspar, with quartz in 

 minor amount. The extinction angles, measured in favour- 

 able sections, range up to about 16°, which, together with the 

 chemical evidence, indicates a fairly pure albite. Ferro- 

 magnesian minerals are rare. There is a little chlorite, 

 pseudomorphic after biotite, and, in one section, there is a 

 little muscovite. The base is typically micrographic, with 

 some spherulitic patches. Calcite occurs in a varying amount 

 in most of the slides examined. Some examples show it 

 arranged along cracks, which have served as lines of percola- 

 tion, and others indicate, in the relic structure still recogniz- 

 able, that the action has been metasomatic. 



One sample, which was practically free from calcite, was 

 chosen for chemical analysis, and the result is interesting in 

 that it indicates a soda-rich type which compares very closely 



