Page:A Treatise on Geology, volume 1.djvu/77

 CHAP. IV. bedded, the beds being separated by thin clays: the marlstone series (d) consists of sandstone beds, calcareous beds, and ironstone bands, separated by thin clays or shales.

The lias formation is included between the Bath oolite formation above, and the red sandstone formation below.

According to this mode of description, the word stratum need never be used as a special term of definition, but reserved for general reasoning. The word series is found to be extremely serviceable in designating a number of similar or similarly associated rocks: the arbitrary word group is also convenient in geological description.

The lamination of rocks offers some interesting facts. Some beds of gritstone (as a) are composed of laminæ parallel to the plane of the beds; such lamination is generally produced by the alternation of mica, whose broad plates cause a partial disunion of the parallel laminæ of quartzose grains. Other beds (as b) are composed of oblique or curved laminæ, a circumstance generally dependent on the irregular admixture of pebbles shells, or particles of unequal magnitude. The former may be supposed to be tranquil, the latter disturbed, deposits.

In shales and other argillaceous rocks, nodules of ironstone or limestone, aggregated round some solid bodies (as a leaf or shell), are frequently included, and sometimes these interrupt the lamination of the