INTERESTING CONTACT OF DOLfclUITE WITH SANDSTONK. 395 



bonate cement having' been replaced by a dark-coloured mass 

 which consists partly of brown mica and partly of an indeter- 

 minate black opaque material. Although the quartz grains 

 have not appreciably diminished in size, examination under a 

 high power shows that each grain is surrounded by a thin 

 shell of secondary crystalline material produced by the fusion 

 of the outer portion of the quartz grains and a combination 

 of the fused silica with some of the components of the cement 

 in which they originally lay. 



It is evident from the above description that the dolerite 

 sill was intruded at a high temperature, but that it was very 

 quickly cooled on being brought in contact with the large 

 mass of cold sandstone. Hence, although in the contact 

 selvage the thermal effects are quite pronounced, the width 

 of the selvage is extremely limited. On the other hand, the 

 cooling effect of the sandstone on the sill itself is very marked, 

 for besides the production of actual selvages, the rapid abstrac- 

 tion of heat appears to have caused the groundmass through- 

 out the whole thickness of the sill to solidify as glass, the 

 felspar and olivine phenocrysts and the isolated felspar micro- 

 lites being- evidentlv of earlier orio-in. 



EXPLANATION OF PLATE XXXI, 



Illustrating Dr. F. H. Hatch's paper, " Note on an Interesting 

 Contact of Dolerite with Sandstone." 



Fig. 1. — X 8. Miei-o-photogvapli of junction of dolerite with 

 sandstone, a. The dolerite " chilled edge." h. The selvage of partially 

 fused sandstone, i-. The iinaltered sandstone. 



Fig. 2. — x 8. Junction of dolerite with sandstone, a. The dolerite 

 " chilled edge." c. The sandstone showing; no '• fusion selvage." 



