Barrell — Physical Effects of Contact Metamorphism. 287 



therefore the only inaccuracies which would attend its use 

 would be from a lack of definite knowledge in regard to the 

 original and final composition of the sediments of any par- 

 ticular case. 



It is seen from the above table that wollastonite is produced 

 from a siliceous limestone, diopside from a siliceous limestone 

 containing some quartz. Vesuvianite commonly has a small 

 part of its alumina replaced by ferric oxide, but here it is 

 computed on an iron-free basis. It is seen to be produced by 

 a sediment not far in composition from that yielding grossu- 

 larite, and has been observed together with it in rock sections. 



In epidote the aluminum and iron are interchangeable, the 

 molecular ratio of the two varying from 6:1 to 3 : 2. For 

 this reason both zoisite, the iron free epidote, and also the 

 alumina free epidote molecule have their relations shown to 

 the original sediments. If it is desired to find what sediments 

 would furnish a given compound of the two molecules, it may 

 be done by considering the ratio which is present in the 

 mineral of the zoisite to iron epidote molecules and their 

 respective molecular weights, that of zoisite being 455, 

 alumina free epidote 451. In the same way any mixture of 

 the albite and anorthite molecules may have its relations 

 determined, the data of each being here given. 



The soda which is frequently shown to exist in hornfels by 

 the presence of a soda-lime feldspar may have existed in 

 unmetamorpbosed strata in a variety of forms. In fresh 

 material it might occur as a soda-lime feldspar or feldspathoid, 

 but these being somewhat readily decomposed it would more 

 naturally be anticipated as a hydrous silicate, especially as a 

 zeolite. Merely to show in a general way the relations between 

 an albite occurring in a hornfels and the zeolite minerals, 

 analcite has been selected. 



Orthoclase, being a mineral which often occurs in minor 

 quantities in hornfelses, has been introduced for the sake of 

 completeness. The greater part of it has probably come from 

 yet undecomj)osed though finely comminuted orthoclase, 

 which under the conditions of metamorphism has collected 

 into definable crystals. Another part, however, is no doubt 

 furnished by some of the many hydrous alkaline silicates 

 reacting with other materials. 



Andalusite is seen to result from a clay upon the expulsion 

 of the combined water, and is attended by the separation of a 

 large amount of free silica, being the only mineral here 

 considered which does not come from the union of two or 

 more minerals of decomposition, but on the contrary breaks 

 into two minerals during the process of metamorphism. 

 Under that part of the table called "Results of Meta- 



