120 



AQUEOUS ROCKS 



rocks. Their ultimate chemical composition is much like that 

 of the clays, and scarce any two samples will show similar results 

 when submitted to analysis. The table given on page 119 shows^ 

 the composition of some schistose argillites used for roofing pur- 

 poses from (I) Harford County, Maryland, (II) Lancaster 

 County, Pennsylvania, and (III) Llangynog, North Wales. 



Shale is a somewhat loosely defined term, indicating struc- 

 tural rather than chemical or mineralogical composition. The 

 word is perhaps best used in its adjective sense, as a shaly 

 sandstone, or shaly limesfone. By many authors it is used 

 with reference more particularly to thinly stratified or lami- 

 nated, clayey rocks. Many shales are but the finer, more fissile 

 portions of sandstone beds; such may represent the off-shore 

 quiet or deep-water portions of arenaceous sediments, which, be- 

 ginning with gravels near the shore-line, become gradually finer 

 as the distance from the shore increases, passing through coarse 

 to finer sands and finally to sandy clays and silts as the water,, 

 through the lessening of its carrying power, lays down its load. 

 Or they may represent later stages in the cycle of sedimenta- 

 tion; the finer silts brought down after erosion have so far 



Chemical Composition of Shales 



OONSTITITENTS 



Silica (SiOs) .... 

 Alumina (AI2OS) . . 

 Iron sesquioxide (FeaOs) 

 Lime (CaO) .... 

 Magnesia (MgO) , . . 

 Potash (K2O) .... 

 SodaCNaaO) .... 

 Sulphur (S) .... 

 Carbon (C) .... 

 Water (HaO)! . . . 

 Phosphoric acid (PaOg) 



97.10% 



II 



50.13% 



72.40% 



66.96% 



10.73 



16.45 



15.626 



5 78 



1.05 



8.38 



0.40 



0.17 



0.493 



1.00 



1.48 



0.677 



« « ii • 



5.08 



3.295 



• • • » 



0.63 



0.628 



4.02 



1.21 



• • • • 



22.83 



Undet. 



3.787 



2.21 



Undet. 



. . * • 



.... 



• * « * 



0.164 



98 37% 





100.00 % 



I. An alum shale from Garnsdorf, near Saalsfeld. II. An alum shale 

 from Barnholm. III. A '^ marly shale " from Breekenridge County, Ken- 

 tueky« 



^ Ignition. 



