THE STUDY OF THE ROCKS 



ft are more likely to decompose than to become 

 fossilized. 



Limestone is easy to obtain and may occupy us 

 for a few moments. We do not wish to go into 

 technical details, but we may say that the word 

 limestone includes a number of rocks which differ 

 largely in appearance and to a considerable extent 

 in composition. It would, perhaps, be more correct 

 to say that there are several kinds of limestone. 

 Some kinds are made up almost entirely of shells 

 and very interesting they are as microscopic objects. 

 One may wonder how a geologist can state with 

 certainty that some spot, may be many miles from 

 the sea, was once covered by salt water. One of 

 these shell-formed limestones may give him the in- 

 formation; he knows that certain shells comprising 

 the rock must have belonged to marine animals, he 

 knows too that whole mountains, which these rocks 

 sometimes form, are not carried bodily on to dry 

 land, so the obvious inference is that the rock was 

 formed below the sea. 



The softer limestones may easily be crumbled and 

 powdered for examination under the microscope; 

 the harder kinds should be treated with acid. When 

 acid is added to limestone a considerable efferves- 

 cence takes place, for the acid decomposes a sub- 

 stance known as calcium carbonate, which the lime- 

 stone contains, and bubbles of gas are given off. 

 When the limestone has ceased to effervesce the 

 portions of the rock which remain, may be carefully 

 washed in water, dried and examined under the 



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