158 



F. ETJTLET ON BEECCIATED POKPIDO-ROSSO ANTICO. 



crystalline, and has a bluish-grey tint, while the cement has a more 

 coarsely crystalline texture, and is nearly colourless. 



Brecciated Porjldo-rosso antico ( x 18 ; by ordinary transmitted 



light). 



The siliceous cetnent is indicated on the margin by the letter C and a line. 

 The distinctly-drawn crystals are chiefly hornblende. 



w^mwM 



¥m 





■O^rf 







41" 



^ri 



FrcuvKH^rU^ . lii 



The difference between the fragments and the cement is still more strongly 

 mai'ked when the section is viewed between crossed nicols. 



"When the section is placed on an opaque white ground and 

 examined by reflected light, the fragments appear of a pale 

 yellowish colour, and their borders, which by transmitted light 

 appeared dark, now exhibit a deep yellowish or coffee-colour. 



On a black ground by reflected light the fragments have a 

 yellowish-red colour with a more intensely red border, in the outer 

 edge of which there is usually a belt of yellowish-white opaque 

 granules. The latter are apparent!}^ kaolinized felspar — similar 

 granules, flecks and altered felspar crystals, occurring irregularly 

 scattered through the interior of the porphyrite fragments. 



The character of the borders of these fragments indicates decom- 



