PLATE XXXIV. 



SPHERULITIC FELSITE, 



THE LEA BOCK | NEAR WELLINGTON, SHROPSHIRE. 



FIG. 1. 



Magnified 10 diameters ; ordinary light. 



The only definitely recognizable mineral represented in the figure is 

 felspar (10). This is mostly striated. The main mass of the rock is composed 

 of perlitic and non-perlitic bands. The perlitic cracks are picked out by green 

 alteration products which are incapable of precise determination. Under 

 crossed nicols the perlitic portions give a micro-crystalline reaction. The 

 non-perlitic bands and patches are coloured red by ferrite. They sometimes 

 show traces of radial structure and when this is the case each radial 

 aggregate behaves as a positive spherulite. Where a band terminates the 

 end is usually swollen. The red colouring matter is disseminated unequally 

 in the red patches and bands. There is always an inner deeply-coloured 

 portion and an outer feebly-coloured portion ; the latter gives only a very 

 feeble reaction under crossed nicols. 



This rock was first described by Mr. ALLPORT (Q.J.G.S., Vol. XXXIII., 

 1877, p. 449). It is older than strata containing upper Cambrian fossils 

 and is therefore presumably of pre-Cambrian age. 



FIG. 2. 



PORTION OF THE ROCK REPRESENTED IN FIG. 1. 



This figure shows the trichites and margarites which occur in the rock. 

 They are best defined in the light red bands. 



FIGS. 3 & 4. 

 PITCHSTONE. 



ARRAN. 



Magnified 100 diameters. Ordinary light. 



The base is a colourless glass. The dusty appearance is due to extremely 

 minute acicular microlites. These are aggregated in places so as to produce 

 feathery forms. That the microlites are hornblende (not augite) is proved 

 by Fig. 4. The larger ones are composed of long narrow laths which are 

 joined together so as to form a hollow crystal having the angles of 

 hornblende. Very often the external boundary of the skeleton crystal is 

 incomplete so that there is a free communication between the glass in the 

 interior and that surrounding the microlites. 



FIG. 5. 



PERLITIC PITCHSTONE. 



ISLE OF EIGG. 



Magnified 20 diameters. Ordinary light. 



The centre of the figure is occupied by a crystal of sanidine. Both 

 glass and crystal are traversed by a crack, evidently formed during the 

 final stages of consolidation. The walls of the crack became separated by 

 contraction more in the glass than in the crystal. The space thus formed 

 became filled ^with a colom'less substance distinct from that forming the 

 main mass of the glass. 



