Handbook of Paleontology 37 



occurs, sometimes in abundance, in thin bedded shales 

 and gives the basis for such names as carbonaceous shales, 

 coal shales, oil shales and black shales. Certain rocks 

 used as hones and known as honestones are composed 

 for the most part or entirely of quartz flour. Lime mud- 

 rocks are mudrocks that have a large percentage of lime, 

 such as the deposits being laid down today in the 

 vicinity of coral reefs. When silica and alumina 

 are added to these, water limes, our natural cement rocks, 

 are formed. 



There are all transitions between clays and shales and 

 they are common rocks in all parts of the world. In the 

 form of slates they are used for roofings, otherwise shales 

 have no value for structural purposes; but they, with 

 clays, have become of value when mixed with limestone 

 in the manufacture of Portland cement which has largely 

 replaced natural cement. 



Limestones and dolomites, as has already been pointed 

 out (page 16), are primarily of chemical or organic 

 origin, largely the latter; and, therefore, in general, 

 whether they contain fossils or not, are a proof of the 

 existence of life at the time when they were originally 

 deposited. Such deposits may be secondarily broken up 

 mechanically and redeposited or taken into solution and 

 precipitated elsewhere. Limestone is soluble in hydro- 

 chloric acid and may be tested for in this way. A drop 

 of acid likewise indicates a calcareous cement in other 

 rocks by an effervescence or bubbling. Dolomite is less 

 soluble than limestone and sometimes the acid must be 

 heated before any reaction can be obtained. Some lime- 

 stones are very pure, consisting entirely of grains of cal- 

 cite. Where there is much clay argillaceous limestones 

 result ; arenaceous limestones, when sand of a silicious 



