38 NATURE STUDY REVIEW [9 : 2— Feb., 1913 



tions are hundreds, even thousands of feet thick. The clays 

 are about the same as the shales in structure. Fossils are very 

 common in shales and are usually well preserved. The grading 

 of shales into other rocks was mentioned above. There are 

 various kinds of shales, some of the more common being: cal- 

 careous, carbonaceous, ferruginous, arenaceous (sandy) and fos- 

 siliferous. Shales are used for making brick and pottery and are 

 mixed with limestone for Portland cement, etc. They are fre- 

 quently called by the name of the product they are used to 

 produce, as brick clay or shale, pottery clay, etc. 



Sandstones and conglomerates differ from each other in the 

 relative size of the materials composing them. When the parti-" 

 cles are larger than a pea (they may be twenty feet or more in 

 diameter) the rock is called a conglomerate (see Fig. 7), when 

 the particles are smaller than a pea it is called a sandstone (see 

 Fig. 8). It can be designated as a coarse, medium or fine- 

 grained sandstone. The sandstone may be in very thhi or in 

 very thick beds, but the conglomerates form a single bed which 

 may be hundreds of feet thick. Cross-bedding, where there 

 are diagonal bedding planes crossing from one horizontal bedding 

 plane to another, is a common feature of sandstones. Sand- 

 stones are deposited near the shore where changes in the 

 direction of the currents are frequent and the cross-bedding 

 is due to these changes. Sandstones are often named from their 

 cement as, calcareous, ferruginous, and argillaceous (clayey) 

 sandstones. They are commonly used for building purposes, 

 more rarely for grindstones and in the manufacture of glass, etc. 



Salt beds are rarely exposed at the surface but their presence 

 is often indicated by salt springs. Salt in beds occurs in New 

 York, Michigan, Louisiana, Ohio, Kansas, Utah and Texas. In 

 some of the states it is more than one hundred feet thick. Gypsum 

 is very often found associated with salt. It is known to occur 

 in beds one hundred or more feet thick and is found in New 

 York, Michigan, Virginia, Iowa, Kansas, California, Texas and 

 many other states. Both these rocks occur interbedded with 

 shales and sandstones and sometimes with limestones. The lime- 

 stone is generally found with the gypsum. When gypsum is 

 found in sufficiently large amounts and pure enough it is used in 

 the manufacture of cements and plaster of Paris. 



Coal is formed by the accumulation of vegetable matter in 

 swamps such as the Dismal Swamp. The water prevents the ma- 

 terial from decaying as it would if it were exposed to the air. 

 Evidence of this rapid decay is seen in every forest. It is esti- 



