COMPOSITION OF METAMORPHOSED SEDIMENTS 195 
more significant than a 13:1 ratio, and ro per cent excess alumina 
having several times the diagnostic value of a 5 per cent excess. 
In attempting to compare in a general way the relative merits 
of various chemical criteria, the writer possibly conveyed a stronger 
impression of their individual worth and potency than he would 
at present desire to do. Few reputable physicians are content to 
diagnose a disease from a single symptom, but when several symp- 
toms all point to a common cause a diagnosis may be made with 
security. To follow the parallel, the chemical relationships used 
as criteria are symptoms which considered singly might result from 
several causes, but taken collectively are known to result from only 
one* cause, rock weathering and assortment. In actual practice the 
geologist is seldom required to depend entirely upon chemical criteria 
but is aided in his diagnosis by structural and textural features 
as well. 
The principal results of the writer’s comparisons of analyses of 
meta-sedimentary with igneous rocks may be summed up in the 
following table in which the figures in parentheses show the number 
of analyses of each class of rocks entering into the comparison. 
TABLE I 
Percentage 
Percentage Percentage Weights of Over 5 Per | Over 10 Per 
Weight of Weight of | Both MgO> |Cent Excess*} Cent Excess* 
MgO>CaO | K.0>Na,0 CaO and Al.O; Al,O; 
K.0> Na.0 
i (79) (74) (74) 
Sedimentary slates...... SAupercent|o2 per cent|78 percent! G24225 9) || s---.- 
Pelite schists and (30) (30) (30) (30) (30) 
PReISSeS ae ere 77 per cent/83 per cent|74 per cent/63 per cent/30 per cent 
Igneous Classes (1481) (1481) (1481) 
rocks of ITand II. .| 8 per cent|363 percent|4} per cent] (1892) (1892) 
Washing- Less than | About 0.6 
ton’s Tables { All other (411) (401) (401) 3 pel cent | percent 
of Analyses \ classes. . .|35 per cent|14 per cent| 7 per cent 
* By “Excess Al.O,;” is meant the alumina in excess over the 1:1 ratio in which it is combined with 
If the “norm” is calculated according to the method of 
the Quantitative Classification of Igneous Rocks, this excess appears as corundum. 
lime and the alkalies in the aluminous silicates. 
The meaning of the table may be made clearer by interpreting 
its third column. 
This column shows that of 74 analyses of sedi- 
A partial exception in hydrothermal metamorphism will be referred to later. 
