169 



series. It occurs, as a rule, in short well formed prisms. Refraction and 

 double refraction are very strong, and the character of the latter is positive. 

 Even the smallest individuals give vivid chromatic polarisation. Professor 

 ROSENBUSCH mentions the occurrence of beautiful crystals of zircon in the 

 Whin Sill of Upper Teesdale. Tourmaline and axinite are sometimes 

 found in basic igneous rocks, where they have been acted upon by granite. 

 The former mineral occurs as columnar crystals. It may belong to the 

 brown or blue variety (indicolite) . The two varieties are sometimes 

 seen to be arranged in a zonal manner in one and the same individual 

 (see fig. I, plate XVII). Tourmaline is distinguished by its strong 

 dichroism, the greatest absorption always occurring in longitudinal sections 

 when the short axis of the nicol is at right angles to the length of the 

 crystal section. Its double refraction is negative. Axinite occurs in 

 certain basic eruptive rocks near their contacts with tourmaline-bearing 

 granites ; as, for example, near Penzance, in Cornwall. It occurs in a 

 massive form as bluish-grey veins in the Chyandower quarry in association 

 with tourmaline, and sometimes also as crystals. 



Pyrites and magnetic pyrites (pyrrhotine) occur frequently as secondary 

 products. The former may occur as crystals (usually cubic) or as irregular 

 masses, the latter always occurs in irregular masses. They are opaque in 

 thin section and can only be distinguished from magnetite by examination 

 in reflected light, when they are recognised by their yellow metallic 

 appearance. The latter is distingished from the former by its magnetic 

 properties, by its bronze-yellow colour in reflected light and by the fact that 

 it is soluble in hydrochloric acid. Copper pyrites is also found as a 

 secondary constituent of basic eruptive rocks. 



Zeolites (1) are frequently found in the rocks now under consideration, 

 but their discrimination is attended with considerable difficulty. Analcime 

 is easily recognised by its form and by its feeble and anomalous double 

 refraction. It is found in association with other zeolites in dolerites of the 

 Car Craig type (see fig. I., plate XX). 



The carbonates of calcium, magnesium and iron are frequently present 

 in basic igneous rocks that have been decomposed by surface agencies. 



They occur in crystalline masses or as a fine crystalline powder scattered 

 through the rock. Their very high double-refraction and the sharpness of 

 their cleavage-cracks are the most useful characters for recognising them 

 under the microscope. In examining slides of decomposed basic igneous 

 rocks it is often advisable to uncover a portion of the section and treat it with 

 dilute hydrochloric acid under the microscope, in order to free it from the 

 carbonates and at the same time to determine the distribution of these 

 substances in the rock. 



One of the most interesting constituents of basic igneous rocks is 

 metallic iron. A method for detecting this substance when present only in 

 extremely minute quantities will be found described by Dr. ANDREWS, in the 

 British Association Reports for 1865. 



(1) A. LACEOIX. Sur le diagnostic des zeolithes en 1'absence de formes crystallines 

 determinables. B.S M. 1885. p. 321. 



