700 A TREATISE ON METAMORPHISM. 



conditions demand diminution of volume, and the large mineral particles 

 are apt to be those of high specific gravity. The more important dense 

 porphyritic minerals are feldspar, garnet, staurolite, tourmaline, andalusite, 

 sillimanite, cyanite, mica, chloritoid, and hornblende. The minerals also 

 exhibit a marked crystal habit, and not infrequently as they grow they 

 maintain crystal forms. Often the porphyritic minerals show a distinct 

 zonal texture, comparable in every way to ■ the zonal texture sometimes 

 shown by minerals in igneous rocks. The zonal texture is frequently seen 

 in minerals having a varying composition, as, for instance, garnet; and is 

 rare or has not been observed in minerals having a rather definite chemical 

 composition, as, for instance, staurolite. It is believed that the zonal texture 

 of the minerals is often due to varying composition, during their growth, of 

 the mineral solutions or adjacent minerals, or both. In some cases there 

 may have been actual cessation of growth and later a renewed growth. In 

 such instances the renewed growth is analogous to the process of enlarge- 

 ment of fragmental mineral particles. 



Single porphyritic individuals may be larger than hundreds of individ- 

 uals of the background. Those a centimeter in diameter are very abundant, 

 while those two or even several centimeters in diameter are not infrequent, 

 and occasionally they are many centimeters across. 



As the large porphyritic individuals form at the expense of many small 

 mineral particles, they either absorb the material of which the surrounding 

 minerals are composed, and thus grow by their destruction, or else absorb 

 a part and inclose a part. The minerals mentioned differ in capacity to 

 absorb the other minerals which before occupied the space they now 

 occupy. In proportion as they lack capacity to absorb the constituents of 

 prior minerals, they have the capacity to grow around and inclose them, so 

 that a single individual may be a large, reticulating, honeycombed mass, 

 which incloses a large amount of other material. (PI. Ill, C.) In some 

 cases the inclosed material may be several times as great in quantity as the 

 host. As minerals conspicuous for their capacity to include other minerals 

 may be mentioned andalusite and staurolite; occupying an intermediate 

 position are garnet and feldspar; while tourmaline usually does not contain 

 any considerable quantity of the prior minerals. 



A large variety of minerals may be included by the porphyritic con- 

 stituents, but the dominant inclusion is quartz. Andalusite and garnet are 



