INTRODUCTION TO THE ROCKS. 35 



this state it dissolves more limestone than it can hold in solu- 

 tion after the pressure is relieved. Hence many springs throw 

 down a calcareous deposit which in the open air hardens into 

 tufa and travertine. It often incrusts mosses and forms what is 

 called "petrified moss." The vegetable matter perishes and 

 disappears by degrees, but the form of the moss remains. 

 Calcareous springs flowing into ponds cause a deposit of chalky 

 matter on the bottom, which is called marl. It is generally 

 mixed with earthy substances washed in from the surround- 

 ing slopes. 



In precisely the same way certain springs deposit peroxide 

 of iron, which is yellowish or red. Iron is also often trans- 

 ported to bogs and ponds, and there undergoes deposition. 

 Thus bog iron ore is formed. 



Now, I wish to ask Johnnie if he understands what has 

 been said in this Talk. I hope he will think these matters 

 over. They will help him to understand, by and by, some 

 matters which are far more interesting. At least, I think he 

 will find them so. 



VI. INTRODUCTION TO THE ROCKS 



KINDS OF MINERALS AND STONES. 



IT is not entirely satisfactory to roam over the fields, with 

 bowlders lying on the right and left, but without any knowl- 

 edge of their names or natures. True, we shall experience 

 much satisfaction in feeling that we know something of their 

 origin and their history. We may walk up to the side of one 

 of these ancient and way-worn travelers and say: "Old Hard 

 Head, when did you arrive in this country, and where did you 

 emigrate from ?" Old Hard Head will lie sullenly and answer 

 never a word. But he is written all over with inscriptions 

 which we can already begin to decipher. So we look on the 

 rounded and weather-beaten form, and say to ourselves: 

 " This immigrant rock came from a northern country. He 



