PLATE ix. 



FIG. I. 

 OLIVINE-DOLERITE. 



TIDESWELL DALE, DERBYSHIRE. 

 Magnified 50 diameters. Nicols crossed. 



This figure shows plagioclase (10), augitc (7), and dark pseudomorphs after 

 olivine (6). 



The sections of augite are large irregular plates. Olivine-pseudomorphs and 

 felspar occur as inclusions. The large patch of augite in the upper part of the figure 

 gives the yellowish-green of the second order. Other portions of augite in the same 

 figure are cut nearly at right angles to an optic axis, and give only the pale neutral tints 

 of the first order. 



The plagioclase, probably labradorite, occurs in lath-shaped sections, and shows the 

 usual twining on the albite plan. It invariably polarises in the neutral tints of the first 

 order. 



No fresh olivine occurs in the slide, but green serpcntinous pseudomorphs, often 

 rendered almost opaque by iron oxide, are abundant. The rock is associated witli 

 carboniferous limestone. It was originally identical in structure and composition with 

 the Tertiary olivine-dolerites of the West of Scotland .(see plate X.) 



FIG. II. 

 OLIVINE-DOLEBITE. 



TIDESWELL DALE, DERBYSHIRE. 

 Magnified 50 diameters. Ordinary light. 



The minerals represented are oli vino (1), plagioclase (10), augite (7), magnetite, and 

 serpentine after olivine. 



Olivine occurs in large crystals and grains. The alteration of the olivine at the 

 margins and along cracks is well seen in the large central crystal. 



The felspar occurs in lath-shaped sections. 



Augite occurs only in the form of granules between the lath-shaped felspar sections. 



The rock is similar to the preceding one in composition, but differs from it in 

 micro-structure. Fig, I. illustrates the ophitic type of dolerite ; Fig. II. the type 

 with granular augite. 



