718 A TREATISE ON METAAlORPHlSM. 



intrusive mass as it cools, following the severe metamorphism, the conditions 

 are those of moderate and mild metamorphism! As the metamorphic wave 

 passes out from the newly intruded batholith the minerals first developed 

 may later be transformed into other minerals requiring more intense 

 metamorphic power, and thus early minerals may be partly or wholly 

 obliterated by transformation into the minerals of more intense metamor- 

 phism. Under such circumstances the facts might be discovered by partial 

 change or by pseudomorphs. The phenomena are finely illustrated about 

 the batholiths of western Massachusetts. According to Emerson — 



The zonal character of the contact metamorphosis around these batholites is 

 interesting, especially in aluminous sediments. The first wave of heat develops the 

 easily formed minerals, fibrolite and chiastolite; stronger heat, staurolite and garnet; 

 then the first influx of the alkaline waters from the granite forms pseudomorphs of 

 these in muscovite, and with increasing heat feldspars develop. So the highly altered 

 rocks nearest the intrusive mass have often passed through all the stages one passes 

 over in going from the outer zone inward. Thus, in the Carboniferous argillite in 

 Harvard one finds masses of interlaced prisms of andalusite, of the largest size and 

 finest pink color, inclosing crystals of fibrolite in abundance (the two not orientated 

 to each other), and the whole in every stage of change to coarse muscovite. This 

 preserves three stages which were plainly passed over in succession, and nearer the 

 granite large feldspars are interspersed. In the Hatfield argillite, a zone of delicate 

 chiastolite is succeeded inwardly by a zone where the chiastolites are changed to a 

 mixture of muscovite and minute twins of staurolite (the mass still retaining the 

 shape and black cross of the chiastolite) by the influence of greater heat and alkaline 

 solutions; and nearer the granite the whole changes to sericite schist, chlorite schist, 

 and finally hornblende and feldspar appear near the contact with the hornblende 

 granite." ' 



In Europe at many localities a zonal arrangement of the metamorphic 

 minerals peripheral to intrusive masses has been observed. The minerals 

 commoidy developed and the order of their arrangement are in general 

 similar to the instances above given. Four well-known instances are those 

 furnished by the Vosges, the Erzgebirge, the Hartz, and the Tyrol. In 

 the Vosges, granite intrudes the Steiger Schiefer.'' Says Eosenbusch : 



Die gesammte Contactzone der Steiger Schiefer an den Granititen lasst sich 

 demnach gemass der wesentlichsten Entwicklungsglieder in 3 anniihernd concen- 

 trische Partialzonen zerlegen. welche etwa zu bezeichnen waren als: 



(1) die Zone der Fleck-, Frucht- oder Knotenschiefer rait unver&nderter ScLie- 

 fergrundmasse oder Knot mthonscl defer; 



"Emerson, B. K., Porphyritic and gneissoid granites in Massachusetts: Bull. Geol. Soc. America, 

 vol. 1, 1890, pp. 560-561. 



h Rosenbusch, H., Die Steiger Schiefer und ihre Contactzone an den Granititen von Barr-Andlau 

 und Hohwald, R. Schultz & Co., Strassburg. 1*77. PP. 17S-250. 



