124 Transaction<i. 



Phenocrysts. — The most numerous and conspicuous phenocrysts are 

 feldspar. These vary in size from 0'3 mm. to 3 ram. They show poly- 

 synthetic albite twinning. The extinction-angle of adjacent lamella? in 

 sections perpendicular to 010 is 28*^'. The feldspar is therefore labra- 

 dorite. 



Less numerous are phenocrysts of grey augite and small pseudomorphs 

 of serpentine with the form of olivine. The augite crystals varv in 

 size from 0-2 mm. to 0-7 mm., and the olivine has been present in crystals 

 1 mm. or 2 mm. in length, and regular grains of magnetite up to 0-5 mm. 

 occur. 



Groimdmass. — The groundmass consists of small laths of feldspar with 

 a considerable amount of fine granular augite and magnetite. The feldspar 

 is labradorite. and the augite is similar to that occurring as phenocrysts. 



Basalt A'o. 5. 



This basalt underlies the Logan's Point phonolite on the Main North 

 Road, near Normanby, where fresh spheroidal cores are obtained, imbedded 

 in a red clay. 



Macroscopic Characters. — The rock appears dense and black, with 

 occasional large phenocrysts of feldspar. It has a very irregular frac- 

 ture. 



Microscopic Characters. — The structure is holocrystalline and porphyritic. 

 The groundmass is very dense, and the feldspar needles appear to have 

 flowed around the phenocrysts. 



i # Phenocrysts. — Feldspar is abundant, in n-regular and lath-shaped crystals, 

 showing polysynthetic albite twinning. The extinction-angle of adjacent 

 lamellae in sections perpendicular to 010 is 27°. The feldspar is there- 

 fore an acid labradorite bordering on andesine. Some of the crystals, 

 especially the lath-siiapcd ones, are tmall; others are as much as 5 mm. 

 in length. 



Grey augite occurs very sparingly, in rounded grains, and there is a 

 small amount of olivine in crystals up to 1 mm. in length. 



Magnetite occurs in grains and octahedrons. 



Groundmass. — In the very dense groundmass feldspar is moderately 

 abundant, in fine needles. The remainder consists of grains of magnetite 

 and augite. 



Order of Crystallization for the Basalts. 



(1) Magnetite; (2) olivine ; (3) augite; (4) feldspar; (5) magnetite, 

 augite, and feldspar of the groundmass. 



Quantitative Classification of the Pochs. 



The rocks have been classified on a quantitative chemico-mineralogical 

 basis according to the system of Cross, Iddings, Pirsson, and Washington,* 

 and the chemical results have also been plotted as diagrams according to 

 the system devised by Iddings.f and these diagrams have been combined 

 in a multiple diagram (Plate IX). 



1903. 



* " Quantitative Classification of Igneous Rocks." Chicago, 1903. 



t Chem. Comp. of Igneous Rocks expressed by Means of Diagrams, U.S. Geol. Surv.. 



