92 STRUCTUflAL GEOLOGY. 



shales ; or conglomerates with shales and coal-beds ; or conglomerates with 

 limestones and sandstones ; or shales and sandstones alone. 



The thickness of each stratum also varies much, being but a few feet in 

 some cases, and hundreds of feet in others ; and the same stratum may 

 change in a few miles from 100 feet to 10, or disappear altogether, or change 

 from one of shale, or of limestone, to one of sandstone, and so on. In the 

 Coal-formation of Nova Scotia there are 15,000 feet of stratified beds, con- 

 sisting of a series of strata mainly of sandstones, shales, and conglomerates, 

 with some beds of coal ; and in the Coal-formation of Pennsylvania there are 

 6000 to 7000 feet of a similar character. 



In many cases a bed of limestone thins out at short intervals, and is thus 

 in isolated pieces, 100 to 1000 feet, or more, long, called lenticular masses, 



shale or sandstone occupying the in- 

 terval. This results from the varying 

 conditions in the seas in which the beds 

 were made, some portions being favor- 

 able for the animals that make shells 

 and other calcareous materials from 

 which limestones are formed, when the 

 larger part is unfavorable. Such lenticular masses {ah, cd, ef, Fig. 60) may 

 consist of iron-ore, such ores being often deposited locally in marshes or 

 shallow basins, on sea-borders, as well as in interior ponds or shallow lakes. 



A seam is a thin layer intercalated between layers and differing from 

 them in composition. Thus, there are seams of coal, of quartz, of iron-ore. 

 Seams become beds, or are so called, when they are of considerable thick- 

 ness ; as, for example, coal-beds. Such seams are sometimes popularly, but 

 wrongly, called veins. 



The beds or layers of rock may be (1) massive, that is, of great thickness 

 without division into subordinate layers ; or (2) thick-bedded, or (3) thin- 

 bedded, or (4) laminated, or (5) shaly. The flagging-stone, much used in 

 Eastern cities of this country, is a good example of a laminated sandstone. 

 Such a variety of sandstone is often called Jiags. 



(6) Straticulate structure is one made up of very thin and even layers, 

 separable or not, as a bed of slate, a bed of clay in a river-valley, stalagmite, 

 and agate. It is often called a banded structure. 



(7) Slaty structure is much like shaly, and frequently a shale is called a 

 slate. But the shale is straticulated parallel to the planes of deposition, and 

 the structure is due more or less to the pressure of the overlying material ; 

 while slate (roofing-slate) has much more even layers, with a smoother 

 surface, and has derived the slaty structure from lateral pressure, as 

 explained beyond (page 112). 



(8) A cross-bedded structure characterizes a layer when it is obliquely 

 laminated, as in three of the layers in Fig. 61. Such layers generally 

 alternate with horizontally bedded layers. This style of bedding is made 

 by a strong movement of a current over a sandy bottom, as in the move- 



