164 



find, as inclusions, the minute tables and rods (see ante p. 29) which give the 

 " schiller," so long regarded as characteristic of these minerals. 



The occurrence of definite inter-growths of rhombic and monoclinic 

 pyroxenes (see ante p. 31) may sometimes be observed. The alteration of the 

 rhombic pyroxenes in the rocks now under consideration is essentially similar 

 to that in the peridotites. A good illustration of the mode of decomposition is 

 seen in fig. 2 plate XIII. Mr. Gr. II. WILLIAMS and Dr. LEHMAXX have 

 described the conversion of hypersthene into compact hornblende. 



Olivine. This mineral plays a most important part in the basic eruptive 

 rocks. It occurs in crystals, grains, and granular aggregates. Well-formed 

 crystals are comparatively rare. When they do occur they give rise to 

 sections which are remarkable for their symmetry. These sections are 

 frequently six-sided, and the two edges which represent the terminal faces 

 meet at an acute angle (see fig. 2, plate X.). The chemical composition of 

 oli vine is represented by the formula (Mg Fe) Si0 3 , and the principal 

 varieties are due to varying proportions of the two bases. In normal 

 olivine the amount of ferrous oxide is generally less than 1C p.c. ; 

 in the variety known as hyalosiderite it rises to 28 p.c. Olivine may 

 be colourless, yellow, or yellowish brown. The depth of colour is an 

 indication of Ihe amount of iron, and generally when the colour is in any way 

 conspicuous the mineral may be referred to hyalosiderite. This is the case 

 with some of the oliviiies in the dolerites of Rowley Regis and the West of 

 Scotland. 



OLIVIXE, FROM THE SUMMIT OF HALIVAL, ISLE OF RUM. f 1 ) 



Si0 2 ... ... ... ... 38-006 



A1 2 O 3 ... 0-286 



Fe 2 3 ... 2-9:33 



FeO ... ... 18-703 



MnO ... 0-100 



CaO ... ... 0-336 



MgO ... 38-000 



H^O 1-587 



Sp. Gr. 



Olivines rich in iron assume a red or reddish brown colour when heated. In some 

 of the olivines in the dolerite near Portrush the central portions are colourless, 

 or nearly so, while the marginal portions are a deep reddish brown. This is 

 probably due to an action of the magma on already formed crystals. The 

 refraction of olivines is very high, and consequently the surface usually 

 appears rough under the microscope. The double refraction is strong, 

 and tints of the third order are frequently seen, even in very thin sections. 

 Olivine in thin section is readily acted upon by hot hydrochloric acid, and this 



(1) Dr HKDDLE. Min. Mug. vol. V. (188-i), p. 1(5. 



