286 Barrell — Physical Effects of Contact Metamorphi 



ism. 



It will be shown later that the losses in volume due to the 

 other two causes may amount to as much as 47 per cent. 

 Thus occasionally the combination of the three may result in 

 a shrinkage of the strata by as much as 50 per cent upon the 

 intrusion of a neighboring igneous mass. The geological 

 consequences of such striking changes may well be studied in 

 some detail. 



Relation of metamorphic minerals to original composi- 

 tion. — From the species and proportionate amounts of the 

 metamorphic minerals seen in thin section, it will be desirable 

 to determine the nature of the original sediments and thence 

 the changes undergone in mass, volume and mineral composi- 

 tion. Leaving aside for the present those possible accessions 

 connected with impregnation and fumarole action, the chemi- 

 cal elements will remain present as before stated except 

 for the expulsion of a greater or less quantity of water and 

 carbonic acid. To determine this relation of metamorphic 

 minerals to original composition, some definite basis must be 

 adopted. For that reason the sediments are assumed to consist 

 of a number of stable minerals, the results of thorough 

 decomposition. As has been shown, in argillaceous rocks such 

 a condition is never perfectly reached, and where such have 

 suffered changes the losses computed on the above basis must 

 be diminished by a factor depending upon the incompleteness 

 of the decomposition of the original rock. In strata consist- 

 ing of quartz sand and carbonates, however, the changes will 

 be strictly those shown by the following table. 



The metamorphic minerals given in the table, except for the 

 omission of biotite, are those of commonest occurrence in 

 strata adjacent to igneous rocks. Biotite, though of common 

 occurrence as a result of metamorphism in rocks of an 

 arenaceous-argillaceous character, has such a complex composi- 

 tion that it is useless to attempt to compute from what 

 materials it has come, unless something is known of the 

 unmodified strata. The ferrous oxide, magnesia and alkalies 

 present in biotite, furthermore, are indicative of an incom- 

 pletely decomposed sediment, and add to the difficulties. The 

 absence of biotite from the table for that reason, however, is 

 not a serious matter, since the materials forming it have 

 possessed but little carbon dioxide and a medium amount of 

 water and consequently in metamorphism have not changed 

 greatly in volume or mass. While the table shows in a general 

 way and with a fair degree of accuracy the changes taking 

 place to produce such minerals ; for any special application it 

 may be necessary to extend it in order to take in materials in 

 the original sediments not here considered. The computations 

 have been carried out with accuracy to the final digit and 



