214 



BOCK BTSTNTEGBATIOK AND DECOMPOSITION 



siderable quantities of undecomposed silicates, and tlioiigh ex- 

 tremely tough and enduring from a human standpoint, in time 

 decompose in a very interesting manner. In the field these 

 rocks are found with their evident cleavage nearly vertical, and 

 forming steep, high ridges flanked by valleys carved from the 

 softer rocks on either hand. In the fresh cuts made during the 

 work of stripping, to open new quarries, the sound rock is found 

 overlaid by a variable thickness of ferruginous residual clay. 

 Joint blocks and splinters of the slate scattered through this 

 clay, in all stages of decomposition leave no doubt as to its 

 origin. Blocks, deep velvety black on the interior, are sur- 

 rounded by a crust of ochreous brown-red decomposition product, 

 the decay penetrating irregularly like the processes of oxidation 

 into a piece of metal. The first physical indication of decay is 

 shown by a softening of the slate, so that it may be readily scratched 

 by the thumb nail, and an assumption of a soapy or greasy feel- 

 ing, the entire mass finally passing over to the deep red-brown 

 unctuous clay, sufficiently rich in iron to serve as a low-grade 

 ochre, for paints. The incidental chemical changes are surpris- 

 ingly large, as shown by the analyses given below, column I 



Analyses of Fbesh and Decomposed Argillite, Harford County, 



Maryland 



CONSTITUKNTS 



Silica 1 (SiOs). . . . . . 



Alumina (AI2O8) . . . . 



Iron oxide (FeO and FeaOs) 



Lime (CaO) 



Magnesia (MgO) . . . . 



Potash (K2O) 



Soda (Na20) 



Ignition (0 and H2O) . . . 



•J 



M 

 O 



to 



44.15% 



30.84 



14.87 



0.48 



0.27 



4.36 



0.51 



4.49 



99.97% 



II 



o 



a 



OQ 



24.17% 



39.90 



17.61 



None 



0.25 



1.24 



0.23 

 16.62 



100.02 % 



HI 









P4 en S 

 « o S 



f^M 



25.34 % 

 0.00 

 1.23 

 0.48 

 0.08 

 3.39 

 0.33 

 0.00 



40.83 % 



IV 



1-4 



to (A 



h. cs --I 



go a» 



o n EH 



w » £ 



w ->i w 



P-t pel P 



42.43 % 



100.00 



91.22 



0.00 



7h84 



22.04 



0.36 



287.37 



o . 



« 2 



2 « H 



^ « 5 



POO 



go *^ 



i o I! 



57.57% 



0.00 



8.78 

 100.00 

 28.16 

 77.95 

 99.64 

 None 



^With traces of TiOa. Manganese in traces, but not determined. No 

 sulphur; hence no pyrite. 



