STORY A SCRATCHED ROCK TELLS 243 



mile thick must exert when it moves over a whole 

 country as it does in Greenland. So long as the 

 snow accumulates faster than it melts, nothing, not 

 even mountains, can stop the ice sheet thus formed 

 as it slowly moves over the surface of the land to 

 the sea. 



Sucli a glacier as that once passed over the north- 

 ern part of our country. In places it was a mile 

 thick, and it covered such high mountains as Mount 

 Washington and the Adirondacks. 



If you ask how we know about such a strange 

 thing as this, we answer as we have answered so 

 many times before, ''Because the rocks tell us;" 

 not from their structure or from the fossils they en- 

 close, but from the marks upon their surface. For 

 they are smoothed over as if a mighty force had 

 been at work upon them, and often those smooth 

 places are scratched with straight parallel lines and 

 grooves. 



''What sort of writing has nature made upon 

 these rocks?" men have asked. "Why should the 

 rocks north of a certain line be so smooth, with long 

 parallel scratches upon them, while those south of 

 that line are just as the rain-water has left them?" 



So people wondered, until a great man named 

 Louis Agassiz, who came to America from Switzer- 

 land, showed us how to read this part of nature's 

 writing. 



He had seen glaciers at work in his native coun- 

 try, for there are many of these rivers of ice among 

 the high Alps, and he had vvatched them carefully. 

 He had noticed the strange appearance of the rocks 



