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 Pig. 4. — Rhy otitic breccia, showing replacement by galena. 



[The galena is seen enveloping a residual fragment of the rock 

 Trecastell Mine. (Transmitted light : X 50.)] 



Fig. 5. — Rhyolitic breccia, Trecastell Mine. 



[The galena has here replaced spherulitic portions of the breccia, which had J 

 been previously silicified. (Transmitted light : X 35.)] 



