716 Elementary Geology.

For in consequence of condensation by pressure, water at the depth of 362 miles, would be asheavy as quicksilver, and air as heavy as water, at 34 miles in depth : while at the center, steelwould be compressed into one fourth, and stone into one eighth of its bulk at the surface. Mrs.Somerville's Connection of the Physical Sciences.

Stratification.

Def. The rocks which compose the globe are divided into two greatclasses, the Stratified and Unstratified,

Def. Stratification consists of the division of a rock into regular masses,by nearly parallel planes, occasioned by a peculiar mode of deposition.Strata vary in thickness from that of paper to many yards.

Def. The term Stratum is sometimes employed to designate the whole mass of a rock, whileits'parallel subdivisions are called beds or layers. The term bed is also employed to designatea layer, whose shape may be more or less lenticular, or wedge shaped, included between the lay-ers of a more extended rock ; as a bed of gypsum, a bed of coal, a bed of iron, &c. In thiscase the bed is sometimes said to be subordinate.

Def. When beds of different rocks alternate, they are said to be interstratified.

Def. A seam is a thin layer of rock that separates the beds or strata of another rock : Ex.gr.u seam of coal, of limestone &c. The term is also employed sometimes in this country to des-ignate the interval or crack between two contiguous beds.

Descrip. A bed or stratum is often divided into thin lamime, which bear the same relationto a single bed, as that does to the whole series of beds. This division is called the laminationof the bed ; and always results from a mechanical mode of deposition. The appearanee of fis-sility which it gives to a rock, is often deceptive; since the layers separate with great difficulty.This is especially true in gneiss.

Descrip. The lamination is sometimes parallel to the planes of stratification ; sometimes theyare much inclined to each other ; and often it is undulating and tortuous.

Fig. 202 shows the different kinds of lamination.

Fig. 202.

Without Laminae.

With waved Laminae.Finely Laminated.Coarsely Laminated.Obliquely Laminated.Parallel Laminae.

Origin of the varieties of Lamination .

Causes- All the lamination of stratified rocks was undoubtedly produced originally by depo-sition in water, and the varieties have resulted from modifying circumstances. 1. The parallellaminae are the result of quiet deposition upon a level surface. 2. The waved lamination, inmany instances, is nothing but ripple marks ; such as are seen constantly upon the sand and mudat the bottom of rivers, lakes, and the ocean. In the secondary rocks this is too manifest to bemistaken. 3. The oblique lamination has generally been the result of deposition upon a steep