THE STRUCTURE AND CLASSIFICATION OF ROCKS. 



627 



jecting ends being exfoliated in such a manner as to present a convex surface down- 

 wards. The Palisadoes, on the banks of the river Hudson, are another example; 

 but the most extensive formations of this kind, perhaps, upon the face of the globe, 

 which appear to throw our Giants' Causeway into insignificance, are in the country 

 west of the Rocky Mountains. There the Columbia river passes through mountains 

 of the trap family, not improbably basalt, which range from 400 to 1000 feet high, the 

 walls consisting of successive rows of columns, superimposed upon one another, sepa- 

 rated by a few feet of amygdaloid, conglomerate, or breccia. In Wales, the columnar 

 structure is developed in the porphyritic trap on the northern side of Cader Idris ; 

 and in England, though obscurely, in some of the basaltic hills near Dudley. This 

 peculiar construction, which distinguishes rocks of the trap family, is supposed to result 

 from a kind of crystallisation while they were cooling down under pressure from a melted 

 state. Mr. Gregory Watt fused seven hundred pounds weight of the Dudley basalt, and 

 caused it to cool slowly, when globular masses were formed, which enlarged and pressed 

 against each other until regular columns were the result. Recent lavas exhibit precisely 

 similar columns. 



The Stratified rocks, which are all of aqueous origin, show a regular divisional structure 

 of layers or beds, to which the Latin word strata is applied, and which have resulted from 

 successive sedimentary processes at the bottom of lakes and seas. They vary in thickness 

 from that of paper to many yards, and are sometimes divided by a thin layer of soft 

 earthy matter, called a seam ; but at other times, the surfaces of the upper and lower 

 stratum are so closely joined, that it requires a considerable force to separate them. If 

 the depositions which constitute this class of rocks went on always in quiet waters, and if 

 there were no disturbing forces in operation, the position of strata would be uniformly 

 horizontal ; but these two conditions have not existed, and consequently the strata are 

 generally found to dip down to some point of the horizon, and of course to rise 

 towards the opposite point. A line drawn through these two points is called 

 the line of their dip ; and another line, drawn at right angles to this, marks Fig , , 6i 



the course along which the strata 

 stretch out to the greatest extent, 

 which is the line of their bearing. 

 Figs. 16. and 17. are examples of 

 horizontal and inclined stratification, 

 showing that an elevating cause has 

 acted upon the latter subsequent to 

 deposition, to which the former has. 



inclined stratification. not been subject With such inten- 



sity has this elevating power operated in some instances, as to lift the strata into a ver- 

 tical position. An 

 interesting ex- 

 ample of this ar- 

 rangement is given 

 in fig. 1 8. by Mr. 

 Murchison from 

 the rock on which 

 Powis Castle . is 

 built. The strata, 



consisting of calcareous red grit, highly charged with portions of encoinital stems, are 

 vertical, or highly inclined at angles of 70 or 80 to the horizon, and appear like massive 

 buttresses erected for the purpose of supporting the noble structure placed upon them, 



88 2 



