MAY AND JUNE 255 



Marble is distinctly crystalline in strncture, a change 

 brought about in its metamorphosis from ordinary lime- 

 stone by the combined action of pressure and heat. 



Its softness and effervescence with dilute acid is the 

 test for all of the limestones. 



The other well-known forms are the ordinary gray 

 limestone and chalk, not the chalk of the schoolroom, 

 which is a paste made up for the most part of sulphate 

 of lime, but the chalk of the White Cliffs of Dover. 



Limestones are all originally aqueous and organic in 

 origin. This is clearly visible to the naked eye in what 

 are know as shelly, coral, and crinoidal limestones. The 

 Coquina stone, now forming along the Florida coast, con- 

 sisting of coarse shell fragments held together with 

 natural cement of lime, tells plainly the story of its 

 formation. 



Sandstones : 



Sandstone may be of almost any color, but it is most 

 familiar to us in the city as " brown stone " and " gray 

 stone." It is granular, hard, gritty, and does not effer- 

 vesce with acids, except the kinds that happen to have a 

 carbonate of lime cement holding the grains together. 

 These last are soft sandstones. 



Brown and red sandstones have a cement of iron, 

 while most of the light sandstones are cemented with 

 silica, which makes, of course, a very hard rock. 



All of the sandstones are of aqueous origin. The 

 stratification is not always visible, but the shape and 

 kind of grains are sufficient indications. Moreover, they 

 often contain fossils. 



The conglomerates, of which the pudding stone is an 

 example, belong to the same family of rocks. They are 



