218 EOCK DISINTEGRATION AND DECOMPOSITION 



bonic aeid in the clay. It will be noted that 87.75% of the 

 entire rock naass has disappeared through leaching, leaving only 

 12.24% to accuimilate as an insoluble residue in the form of soil. 



A compact, deep blue-gray limestone belonging to the Trenton 

 period and occurring near Hagerstown, Maryland, leaves a deep 

 red, clayey soil, poor in lime and containing but the less soluble 

 constituents of the parent rock. Subjected to analyses and 

 calculations as above, this showed a total loss of materials 

 amounting to 98.75%. Another sample (a magnesian limestone 

 from near Staunton, Virginia) suffered a loss of but 90.76%, and 

 showed some mmor differences which it may be well to note in 

 detail. The residual clay is of a deep-red color, highly plastic, 

 and on drying becomes so hard as to be broken only by means of 

 a hammer. 



So abundant is the iron oxide that the mineralogical nature of 

 the residual material is stained almost beyond recognition, and 

 it is only when it is first boiled in dilute hydrochloric acid to 

 remove the iron, that it can be studied at all satisfactorily. 

 When thus treated and submitted to microscopic examination, 

 it is found to consist mainly of very irregularly rounded and 

 angular quartz fragments, which are more or less corroded and 

 unmistakably of clastic origin, i. e., they existed in the limestone, 

 not in the form of particles crystallized in place, but as me- 

 chanically included detritus formed from the breaking down of 

 pre-existing siliceous rocks. Particles of feldspar, a portion of 

 which show twin banding, are also present and, more rarely, are 

 found shreds of white and black mica, chlorite, epidote, and, 

 very rarely, a minute but very perfectly preserved, doubly ter- 

 minated colorless crystal, with forms characteristic of rutile. 

 Both quartzes and feldspars are roughened and corroded, though 

 even the plagioclase feldspars are still in many cases sufficiently 

 fresh to show twin striae. The analyses and calculations based 

 thereon are given on the next page.^ 



It is to be noted that in this case the magnesia and lime were 

 lost in nearly equal proportions. The fact that so large a pro- 

 portion of the alkalies remain in the clay is due to their being 

 constituents of the silicate particles which did not decompose as 

 rapidly as the lime and magnesia were removed in solution. 



^Analyses recalculated on basis of 100 and all tlie iron considered as 

 feme. W^ith the silica is included 0.09% TiOa, See Bulletin 150, U. S. 

 Geological Survey. 



