XII. — Maguified sections showing alteration in Basalt. 



Breccia of an original glass, whose interstices have been fill.'.l with 



ferrnginous clay from the lake bottom. 

 Incipient stage of Zeolitic alteration of glass, showing effect' of chill 



cracks in directing percolation of sohitions. 

 Alteration effects in brecciated glass. 

 Prominent banding in secondary products, dne to cracks in the original 



glass. 

 Geometrical patterns in garnet, interpreted as dne to resorption of 



olivine. 

 I)icipient alteration of glass, showing accentuation of physi.-al and 

 chemical differences by a slight degree of alteration. 

 XIII.— Magnified section of Secondary Minerals in Basalt. 



Association of Garnet CGr), fibrous green Ampliiliole (Am). A11)ite 

 (Ab), Prehnite (Pr), Specularite (Spj, Chabazite (Ch.i, and Calcite 

 (Ca). forming a crust on dense Basalt. 

 Association of Garnet, Amphibole and I'rolmite. 



Complex forms of Secondary :\rinerals found in an advanced stage of 

 alteration of glass. 

 XI^^^ — Outcrop of Bourran Coal Bed at Firmy. 



Southerly end of the Decazeville Decouverte. 

 XV. — A "local fault" in the easterly wall of the Decazeville Decouverte. 

 The Domergne Decouverte. 

 XVI.— Portrait of R. I*. Whitfield. 



Text FifitircM. 



Page 



CCritliilliil tlthcirjSlDll !•' 



Microlitic growth of phenocrysts in vitrophyric crusts PK) 



Chloritie nodides from resorption of olivine P'- 



Basalt bordered by vein albite, prehnite and natrolite l-l 



Single crystal nf secondary all>ite. showii'g both carlsbad and albite tvin- 



ning 



Crystal of secondary albite partly replaced by ])ect()lite 124: 



Iieplacement of albite by iirelmite l-~ 



Keplacement of albite by natrolite 3-8 



Fragments of quartz outlining tDrmei- crystal. ni>\v alnmst cdmpletely re- 

 placed by datolite P">'5 



Fragments of quartz with prehnite being replaced by apoiihylite P".4 



<,tn;irtz cut iiy vein of clial)azite and corroded by same en margin P!4 



Peiilacement (if (pnirtz ( clear) by chabazite ( blocky I loo 



Replacement of ijuartz (clear) by heulandite (stipjiled > l-^''""' 



Replacement of quartz l)y calcite 1-'>T 



Shreds of an abnoi'inal amidiibole (jirobably arfvedsonite) in calcite 141 



Association of :imphibole, datolite. chabazite, stilbite and calcite 14."( 



Remnants of datolite crystals i)erched upon prehnite in contact with stilbite l.")(i 



Corroded crystals of prehnite in the midst of datolite lol 



Datolite replaced by chabazite and calcite l-"- 



Replaceinent of datolite by stilbite l")-"! 



Sw.-irms of corroded crystals of datolite in stilbite l"''-5 



vi 



)•) 



124 



