ASSOCIATED ROCKS OF THE LIZARD DISTRICT. 907 



red variety ; for towards the outside it exhibits perfectly the change 

 into the red variety. I have specimens about 4 inches long, red at 

 one end and green at the other. Hence the alteration may not be 

 so deep-seated as from its uniformity I should have supposed. 



I have examined three slides of the red, cut from slightly different 

 varieties of the rock, and one of the green. I will refer to them by 

 numbers, taking the most normal specimen first. 



On placing slide I. beneath the microscope, we find it to consist of 

 colourless or nearly colourless felspar, in rather irregular to roundish 

 oblong grains, occasionally showing lines indicative of twinning, 

 traversed often by cracks and in places partly kaolinized. Associated 

 with this, in about equal quantities, are a large number of irregular 

 grains of olivine ; these in parts are almost unaltered, in other 

 parts entirely converted into serpentine. This is occasionally 

 translucent and of a greenish yellow, occasionally opaque, from the 

 presence of a muddy brown peroxide of iron, and showing every 

 grade of intermediate staining. The process of conversion of the 

 olivine into serpentine will be described below. 



Fig. 6. — Shore below Coverack Cove. 



b ES3 



Newer gabbro intrusive in older gabbro, and both cut by dyke of dark trap. 

 a. Older gabbro. b. Newer gabbro. c. Dark trap. 



With polarized light (crossed prisms) the felspar is seen to be 

 crystallized in irregular grains, many of which show characteristic 

 plagioclase twinning ; bright colours, however, are rare, shades of 

 light and dark milky grey being commonest. In parts the felspar 

 is almost opaque from decomposition ; i.i other parts it presents the 

 usual finely granular " saussuritic " aspect. The olivine, when un- 

 changed, shows its characteristic rich colouring. The process of con- 

 version into serpentine, best examined by rotating the polarizer, 

 is as follows (see figs. 8 & 9). In the cracks of the olivine a dark^ 

 ferruginous stain is deposited ; then on either side of this a layer of 

 fibrous serpentine of pale golden colour (probably chrysotile) is 

 formed ; thus the olivine grains seem traversed by an irregular net- 

 work of associated dark and light strings. The interspaces then 

 seem to be attacked ; and they also are converted into serpentine ; 

 but in them the mineral is usually in an isotropic or noncrystalline 

 state, and the peroxide of iron either forms a dark clot in the middle 



