ROCK MAKING 211 



Did you ever see any rock called pudding-stone? 

 That is what the pebbles made when the thick mass 

 of them, lying by the ocean's edge, was pressed with 

 such force that it was cemented into a solid rock. The 

 sand was pressed into sandstone and the clay into 

 slate. 



All through the world immense deposits of sand- 

 stone and slate are found, but the conglomerate rock 

 is not very common. It is often called Roxbury pud- 

 ding-stone, and it is used for buildings. Sandstone 

 is used for trimming buildings, and often for the build- 

 ings themselves; also for paving sidewalks and street 

 crossings. Slate is most useful for covering roofs, 

 although it is used in many other ways. 



You would not have thought when you saw that 

 piece of slate that it was made under the bottom of 

 the sea, out of particles of older rocks that the rain 

 water had washed off, and the brooks and rivers had 

 carried into the ocean. 



You would not know, until you were told, that the 

 sandstone was made in the same way. Yet if you 

 look very closely at a piece of sandstone you will see 

 the grains of sand plainly enough, and it is quite easy 

 to see the rounded water-worn pebbles in the pud- 

 ding-stone. 



So all the material that the rain water had been 

 wearing from the earth and carrying to the sea was 

 changed back into rock, and in time raised to the 

 surface of the earth, there to be worn off again. 



Other rocks besides these were being made with the 

 help of the percolating drops of sea water, but tlie 

 particles that went to make thorn had never been 



