EXERCISES ON PHISICAL GEOLOGY. 159 



the most celebrated. Objects exposed to the action of these 

 waters become incrusted with a coating of carbonate of lime, 

 and in this state they are improperly termed petrifactions, 

 since they are merely incrusted with the stony matter, not 

 permeated by it. For example, if we place any organic sub- 

 stance, as a piece of wood or bone, in an incrusting spring, 

 we find that the object is merely coated with the stony 

 matter, but internally is wood or bone still ; whereas in real 

 fossils, not only the mere external covering, but the whole of 

 the interior structure, is converted into stone. 



ATJTHOES. The objects comprised in the term Physical 

 Geology are so numerous and diversified, that we must again 

 refer to the authors already named for more ample informa- 

 tion. They may be severally consulted with advantage, for 

 the following purposes : For the elements and combinations 

 of rocks, read Dr. M'Culloch's Classification of Rocks, his 

 System of Geology, and his Western Isles of Scotland ; as 

 also the Introduction of Mr. Bakewell, and the Guide and 

 other works of Professor Phillips : for the action of water in 

 currents, streams, rivers, lakes, and seas, and the dynamics of 

 the subject, the writings of Sir H. T. Delabeche, Dr. Hutton, 

 and Professor Playfair, may be studied ; while the works of 

 Sir C. Lyell will be found replete with instruction in every 

 department of geology. 



EXEECISES 

 ON PHYSICAL GEOLOGY. 



1. Commit to memory the fourfold division of rocks enu- 

 merated at the commencement of the chapter. 



2. Impress on the recollection, the most important cha- 

 racters and distinctions of the primary rocks, for the reason 

 that they are less clearly defined than those of the sedi- 

 mentary deposits. 



3. Commit to memory the names of the successive forma- 

 tions, together with the distinctive features of each, under 

 the heads of mineralogical character, order of succession, and 



