JlJjo 



AQUEOUS BOCKS 



In chemical composition the limestones, like other sedimentary 

 rocks, vary greatly. As a general rule, those varieties, which 

 have been formed in deep waters and at a distance from the 

 shores, will be of greatest purity, since less likely to have be- 

 come contaminated through detritai materials washed in from 

 the land. Even these may, however, be intermingled to a very 

 considerable extent with the fine siliceous and ferruginous mat- 

 ters such as deep-sea dredgings have shown to be common to 

 modern sea-bottoms. The table on page 127 will give some idea 

 of the wide range in chemical composition found in rocks of 

 this class. 



The name shell marl, or merely marl, is given to an illy defined, 

 often arenaceous, soft and earthy rock consisting essentially of 

 shell material in a more or less fragmental condition, and usu- 

 ally intermixed with more or less clayey matter or siliceous 

 sand and silt. Geikie^ would limit the term to fresh-water 

 accumulations of remains of moUusea, entomostraca, and fresh- 

 water algse, but unfortunately the word has not been so used 

 in much of the literature extant. These marls, being easily 

 decomposed, and on account of their occasional richness in 

 phosphoric acid, or, perhaps, merely on account of the lime 

 they contain, are of value as fertilizers. The analyses below 



Chemical Composition op Marls 



COSTSTlTXTEirTS 



Silica (SiOg) 



Oxide of iron and alumina 



(^A.iaVjg'' Jj ©2^3/ • • • • 



Lime (CaO) 



Magnesia (MgO) . . . . 



Potash (K,0) 



Soda (NajO) 



Phosplioric acid (P2O5) 

 Sulphuric acid (SO3) . . 

 Carbonic acid (CO2) . . 

 Organic matter and water 

 (C and H2O) .... 



Totals 



I 



II 



ni 



IT 



Y 



VI 



% 



% 



% 



% 



% 



% 



6.97 



61.61 



18.84 



58.25 



25.28 



39.36 



0.86 



2.80 



2.72 



11.28 



3.02 



3.47 



47.62 



19.60 



41.48 



13.49 



37.52 



28.96 



1.03 



• * * • 



. • . • 



«... 



0.12 



0.16 



0.37 





0.56 



• « 



* * 



0.22 



0.75 



0.15 



* • * a 



0.09 



• ■ • 



1 » 



0.25 



0.17 



0.19 



• « • « 



0.18 



. . 



p ♦ 



0.40 



0.11 



0.41 



0.06 



0.64 



. . 



m 



40 



0.18 



38.15 



15.37 



32.07 



10.69 



29.02 



22.73 



4.25 



* * * « 



3.42 



« • • • 



2.98 



4.11 



99.00 



99.44 



100.00 



93.61 



99.21 



100.00 



VII 



% 



5.65 



3.30 



48.51 

 1.95 

 0.23 

 0.30 



trace 

 0.31 



39.80 



0,60 

 100.66 



^ Text-book of Geology, 3d ed. 



