88 SIMILARITY OF STRATA OF DIFFERENT AGES. 



Ingredients of Mechanical Strata. Considered with reference to 

 the nature of the ingredients which compose them, mechanical strata 

 form another scale. 



Thus gneiss, one of the oldest of these strata changed by meta- 

 morphism, is a compound of the same ingredients as granite quartz, 

 felspar, and mica ; but these minerals, instead of being amalgamated 

 (so to speak) together by crystallisation, are accumulated in successive 

 laminaB more or less regular, and more or less soldered together. Some 

 varieties of gneiss, therefore, differ from micaceous sandstone less 

 than is commonly imagined, and often other varieties occur which 

 have so slight a lamination and so much of crystallisation as to 

 justly bear the name of granitic gneiss. 



Sandstone sometimes contains rolled and broken pieces of crystal- 

 lised felspar, such as that which occurs in the granite of Cumbria and 

 Scotland. There is, therefore, every reason to conclude that coarse 

 sandstones, like the Millstone Grit, have been derived from the waste 

 of ancient tracts of granite and metamorphosed rocks. Nearly all 

 sandstones contain a small amount of felspar. 



Sandstones sometimes extend over vast districts, and are char- 

 acterised by some remarkable mineral ingredient ; as, for instance, 

 the Green-sand of England, France, and Switzerland, which is dis- 

 tinguished by the presence of a peculiar green mineral, termed Glau- 

 conite or silicate of iron. 



Conglomerates, on the other hand, are generally constituted of 

 fragments from the neighbouring mountains or coast. Thus the red 

 sandstone of the Vosges mountains contains quartz pebbles derived 

 from the slate rocks of the vicinity ; the Old Red conglomerate of 

 England varies in composition according to its locality ; that of 

 Herefordshire contains much quartz. 



Whole Series of Strata. The whole series of stratified rocks, 

 then, consist of alternate deposits of limestone, sandstone, and clay, 

 with a few layers of coal, rock-salt, flint, iron ore, &c. The modes 

 of alternation are different in different parts of the series and 

 in different situations. Thus the Siberian limestones are some- 

 times enclosed between beds of slate, the Carboniferous Limestone 

 alternates with sandstone and shale, the Lias limestone lies in marly 

 clays and shales, the Coralline Oolite alternates with calcareous sand- 

 stone. Generally, the different strata are distinguishable by their 

 mineralogical characters, but not always. When the circumstances 

 of the deposit were nearly similar, as in the accumulation of the 

 Carboniferous Limestone and some of the oolites, the strata are 

 remarkably alike ; and often particular beds of one rock are scarcely 

 to be distinguished from beds of another rock. The Old Red Sand- 

 stone and the New Red Sandstone formations are physically very 

 much alike ; it would be difficult by mere mineralogical methods to 

 discriminate the great clay deposits which separate the oolitic lime- 

 stones, and many sandstones of very different epochs are almost 

 undistinguishable from each other. Hence we may infer that nearly 

 the whole series of strata is the result of many repetitions of similar 



