FORMS OF STRATIFICATION. 
39 
ing already combined with sulphuric acid, for which it has a 
stronger affinity than for any other. Anhydrous gypsum is 
a rare variety, into which water does not enter as a compo¬ 
nent part. Gypseous marl is a mixture of gypsum and 
marl. Alabaster is a granular and compact variety of gyp¬ 
sum found in masses large enough to be used in sculpture 
and architecture. It is sometimes a pure snow-white sub¬ 
stance, as that of Yolterra in Tuscany, well known as being 
carved for works of art in Florence and Leghorn. It is a 
softer stone than marble, and more easily wrought. 
Forms of Stratification. —series of strata sometimes con¬ 
sists of one of the above rocks, sometimes of two or more in 
alternating beds. 
Thus, in the coal districts of England, for example, we oft¬ 
en pass through several beds of sandstone, some of finer, oth¬ 
ers of coarser grain, some white, others of a dark color, and 
below these, layers of shale and sandstone or beds of shale, 
divisible into leaf-like laminae, and containing beautiful im¬ 
pressions of plants. Then again we meet with beds of pure 
and impure coal, alternating with shales and sandstones, and 
underneath the whole, j)erhaps, are calcareous strata, or beds 
of limestone, filled with corals and marine shells, each bed 
distinguishable from another by certain fossils, or by the 
abundance of particular species of shells or zoophytes. 
This alternation of diferent kinds of rock produces the 
most distinct stratification; and we often find beds of lime¬ 
stone and marl, conglomerate and sandstone, sand and clay, 
recurring again and again, in nearly regular order, through¬ 
out a series of many hundred strata. The causes which may 
produce these phenomena are various, and have been fully 
discussed in my treatise on the modern changes of the 
earth’s surface.* It is there seen that rivers flowing into 
lakes and seas are charged wdth sediment, varying in quan¬ 
tity, composition, color, and grain according to the seasons; 
the waters are sometimes flooded and rapid, at other periods 
low and feeble; difierent tributaries, also, draining peculiar 
countries and soils, and therefore charged with peculiar sed¬ 
iment, are swollen at distinct periods. It was also shown 
that the waves of the sea and currents undermine the cliffs 
during wintry storms, and sweep away the materials into 
the deep, after which a season of tranquillity succeeds, when 
nothing but the finest mud is spread by the movements of 
the ocean over the same submarine area. 
It is not the object of the present work to give a descrip- 
* Consult Index to Principles of Geology, ‘^Stratification,” “Currents,” 
‘ ‘ Deltas, ” “ Water, ” etc. 
