88 GEOLOGY OF THE SON VALLEV, ETC, 



silky patches of chlorite which are found to be pseudomorphs after 

 olivine. The specific gravity is 3*03. The microscopic section 

 shows idiomorphic porphyritic crystals of augite and of the 

 altered olivine scattered through a minutely fine-grained matrix ex- 

 hibiting typical basaltic structure. The augites, like those of the rock 

 ^77, belong to the pale-coloured variety and they also appear very 

 transparent in the hand-specimen. Some of them seem to have been 

 corroded by the magma. Again as in T \\ the augite of the ground- 

 mass is altered to minute hornblende needles. The felspar is, how- 

 ever, quite unaltered, and the shape of the minute microlites which 

 it constitutes, very well preserved. The base also contains chlorite 

 and epidote. Ilmenite is scattered throughout the base in minute 

 grains as is seen in many typical basalts and it is only partly altered 

 to leucoxene. The groundmass of the rock is very similar to that 

 of T W f rom Girwi, described on page 80, only without the larger 

 porphyritic felspars. The most interesting feature is, however, the 

 olivine, porphyritic crystals of which constituted originally a large 

 proportion of the rock. It is entirely altered to chlorite, but the 

 outlines of the crystals are admirably preserved. A feature worthy of 



Curious pseudomorphs n ° tice is that the °P tic *™ of the chlorite 



of chlorite after olivioe. coincides with one of the crystallographic axes 

 of the original olivines and the pseudomorphs give perfectly 

 straight extinction. Sometimes the totality of the chlorite is 

 developed parallel to one axis of the olivine, or else a smaller 

 proportion coincides with another axis at right angles to the 

 first one, in which case a series of meshes is produced which 

 probably represents the original cracks in the olivine. This alter- 

 ation might be compared to the change of enstatite into bastite. 

 Detached flakes of the chlorite give the characteristic uniaxial figure 

 of that mineral in convergent polarised light. The mineral 

 exhibits a bright pleochroism from green to pale yellow, and the 

 pseudomorphs form remarkably beautiful objects under the 

 microscope. 

 ( 88 ) 



