teheanes. 91 



region has remained for ages as permanent dry land, without interior seas, 

 little or no deposits have been made over the surface ; and the little has 

 come through the winds or rains or igneous ejections. So, also, where the 

 deep oceans have been located, the deposits have been relatively thin. The 

 earth's coat of stratified material is hence a very irregular and ragged one. 



In the description of a formation, the term stratum (from the Latin for 

 bed, strata in the plural) is used for each section of the formation that 

 consists throughout of approximately the same kind of rock-material. Thus 

 if shale, sandstone, and limestone succeed one another in thick masses, each 

 is an independent stratum. A stratum may consist of an indefinite number 

 of beds, and a bed, of numberless layers. But the distinction of layer and 

 bed is not always obvious. 



The series of formations in the earth's structure is divided into series, 

 groups, sub-groups, and stages, according as breaks in the history of higher 

 and lower grade may require. The sei^ies are the grander divisions ; e.g., 

 the Devonian series, the Carboniferous series. The study of the succession 

 of strata or of beds in the rocks of a region, in order to ascertain their origi- 

 nal order and the characteristics of the beds, is a stratical or stratigraphical ^ 

 investigation. The following are some illustrations : — 



Fig. 59 represents a section of the strata as exhibited along Genesee 

 E-iver, at the falls near Eochester. The height of the section is 400 feet. 

 (1) The stratum at bottom is sandstone ; next above it (2) lies a hard, gray 

 stratum, which has been called the G^-ay 

 Band. On this rests (3) a thick stratum of 



greenish shale, fragile and imperfectly slaty ; 5^ ■ '^g 



and (4) a compact limestone. Above this j .^.^^^^^^^^^^^^^^^^^^^g 



(5) is another greenish shale, much like that I q ^-.s^^^^^^^^ ^B 



below; then (6) another great stratum of ^i\ y^^^^^^^^^^^§ 

 limestone; (7) another thicker stratum of ~ 



shale; and, finally (8), at the top, is limestone wholly different from those 

 below. The transition from one stratum to another is quite abrupt ; and, 

 moreover, each may be traced for a great distance through the adjoining 

 country. It must be here remembered that these transitions in the rocks 

 indicate extended changes in the conditions of the rock-making seas ; that 

 when a pure limestone was in progress, the sea was free from currents 

 bringing in mud or sediment ; when making shale, the currents carried in 

 fine sediment ; when sand, a coarser sediment ; so that alternations in depth, 

 limits, and exposure to waves and currents, or not, through the successive 

 periods, were the source of the alternations in the strata. 



The succession of strata in stratified rocks is exceedingly various. In 

 other sections, as at Trenton Falls, IST.Y., there are only limestones in sight ; 

 but, were the rocks in view to a much greater depth, sandstone would be 

 seen. In still other regions, there are alternations of conglomerates and 



1 The latter adjective is a mongrel word, half Latin and half Greek; but it has probably 

 been too long used to be displaced. 



