284 Barrell — Physical Effects of Contact Metamorphism. 



oxide being limonite. The ferrous oxide, magnesia, lime, 

 titanium oxide, and phosphoric pentoxide may also exist more 

 or less fully hydrated. 



Computing on this basis how much combined water would 

 exist in the clay of analysis B if it were fully decomposed and 

 hydrated, the following is obtained : 



Per cent of elements Per cent of water, 

 in clay. Anal. B. if fully hydrated. 



A1 2 3 .. 19*92 1-0 



Fe^O, 7-30 1-2 



FeO 

 CaO 

 MgO y 2-37 0-5 



PA | 



TiO, J 



H 2 above 110° found. 6*52 required 8*7 



From this table it is seen that in this clay, subjected as it 

 has been to most extreme conditions of weathering, the con- 

 stituents are not more than three-fourths fully hydrated. 



The same conclusion may be reached by looking over 

 various reports on clays. The composition of kaolinite is : 

 silica 46*5, alumina 39*5, water 14*0, the ratio of water to 

 alumina being 35 per cent, yet it is seldom that more than 80 

 per cent of the full amount of combined water is present. 



The Limits of Decomposition. — To sum up the foregoing 

 discussion it may be stated that, disregarding the infiltration 

 of elements from without, the quartz sand present in a sedi- 

 ment exists in its original state : the amounts of alkalies 

 present are a measure of the incompleteness of decomposition, 

 since upon complete weathering they are removed. If alkalies 

 are present, some of the lime may be combined with the soda as 

 a plagioclase feldspar, and the magnesia, in a similar manner, 

 may exist in combination with the potash as a mica. In cer- 

 tain hydrous silicates small amounts of lime and magnesia 

 may be also combined with alkalies. But beyond these small 

 amounts they exist as carbonates and their final stage of 

 alteration has been reached. The alumina is not usually more 

 than three-fourths fully hydrated to kaolinite. 



From these statements it is seen that the amount of com- 

 bined water may be taken as an indicator of the completeness 

 of weathering. 



Changes of Mass and Volume in Metamokphism. 



Expulsion of constituents and loss of mass. — It is commonly 

 observed that metamorphosed strata are composed of certain 

 mineral species, most of which show habits characteristic of 

 these rocks. 



