462 EDSON S. BASTIN 
removal in solution. The solubility of these carbonates is dependent 
upon a large number of factors, but in a general way calcium carbonate 
is more readily soluble than magnesium carbonate. Ina rock, there- 
fore, which contains both lime and magnesia, the lime is usually 
removed more rapidly than the magnesia and the relative amount of 
magnesia shows a progressive increase. 
Since this selective removal of carbonates is largely effected 
through the agency of circulating waters it is a phenomenon more 
characteristic of the upper zone of the lithosphere than of the deeper 
zone in which secondary foliated structures are developed. There 
appears to be little evidence that the processes which have produced 
the foliated structures in most of the metamorphic rocks have effected 
any great changes in the relative proportions in which CaO and MgO 
are present in the rock, the tendency under these conditions being 
for these oxides to combine in the more stable form of silicates. If, 
for example, we compare the percentages of lime and magnesia in 
the composite analysis of 51 Paleozoic shales made in the laboratory 
of the U. S. Geological Survey’ with the percentages shown in the 
table on p. 456 for the averages of the slate and pelite schist analyses, 
we find that the relative proportions are closely similar in the three 
groups. ‘The percentage weights corrected for the water content 
are given below: 
MgO CaO 
(i) Shalesecehee cee oie ae 2.43% 1.48% 
(yeSlates; nicky thos. ean ee ieee 200 Fea 
(3) Schists and gneisses... .. ie 2.47 1.53 
It appears clear from these comparisons that dominance of mag- 
nesia over lime is a feature developed in the pelitic sediments, not 
during dynamic metamorphism but during the processes of rock 
disintegration and decay. 
A comparison of the pelitic foliates with the igneous rocks as 
tabulated in Washington’s tables gives the relationships with respect 
to magnesia and lime shown in the first column of the table below. 
Among the 79 analyses of sedimentary slates which enter into the 
average given in the table on p. 456, MgO >CaO in 84 per cent., while 
among the 30 pelite schist analyses whose average was given in the 
t F. W. Clarke, Bull. 330, U. S. Geol. Survey, p. 468 (1908). 
