Barrell — Physical Effects of Contact Metamorphism. 291 



proving the addition by circulating waters of any basic 

 elements. Pure limestones in the same vicinity and but four 

 feet from the granite mass are converted into white, coarsely 

 granular marbles, whose only impurities are 5*0 per cent of 

 wollastonite in scattered microscopic crystals and 4*5 per cent 

 of some other mineral now completely decomposed. This 

 indicates the presence of not more than 5 per cent of silica in 

 the marble and shows that no appreciable addition to it has 

 taken place. 



As these are rocks, however, from which no liberation of 

 gases with shrinkage has taken place, a better test is to observe 

 the changes in the Cambrian limestone, Starmount formation, 

 of the same district. These rocks where metamorphosed have 

 an average composition as follows : 



Per cent. 



Grossularite garnet „ 60* 



Diopside _ 25 • 



Wollastonite __ _ 15* 



100- 



This gives a total silica percentage to the rock of 47 per 

 cent. The garnet consists of grains *05 mm diameter, clus- 

 tered together in areas and also scattered through the 

 intervening minerals, colorless in plain light and taken to be 

 grossularite, though the pyrope molecule may occur to some 

 extent. 



The diopside is later than the garnet and is ophitic 

 over areas of from 5 to 10 mm . Thirdly, the wollastonite, 

 youngest in generation, is ophitic in similar manner to the 

 diopside. The rocks are dense and even-grained and without 

 cracklings or infiltrations of quartz or calcite. The thin sedi- 

 mentary banding is still preserved with the same lenticular, 

 somewhat concretionary structure observed at a distance from 

 the igneous intrusions, and the adjacent layers, where of 

 different mineral composition, are sharply separated from each 

 other. Certain strata may show a few per cent of calcite, but 

 this is distributed in a manner which indicates that it is not a 

 secondary addition, but on the contrary is due to those beds 

 containing originally more calcium carbonate than could com- 

 bine under metamorphic action with the quartz and kaolin 

 present. These features sharply separate the mass of Star- 

 mount strata from certain beds, which owing to special 

 conditions do show infiltrations, as indicated by nuclei of 

 quartz with fluorite and ore grains. 



The metamorphism in rocks such as the Starmount siliceous 

 and argillaceous limestones is strongly marked for distances of 



