46 



PRINCIPLES OF STRATIFICATION. 



as a more extended survey of the district would have shown him, 

 that the slate rocks rest on other beds, which have the same inclina- 

 tion as the limestone above them, and that the slate and hmestone are 

 conformable. 



The modes of stratification which we have been considering are 

 those of plane strata ; but, in many situations, particularly in the Alps 

 and the Jura chain, the strata are curved and bent round the moun- 

 tains, encircling them like a mantle. The ravines and escarpments, 

 according to the position in which the sections have been made, pre- 

 sent the most varied forms of stratification in the same mountain. In 

 one part, the strata will seem to rise almost vertically ; in another, to 

 be nearly horizontal ; and in a third, to be deeply curved : and this 

 will depend, much, on the relative position of the observer, whether 

 he be placed on one side, or in the face of the escarpment. Sup- 

 pose a transverse section to be made through a mountain in the direc- 

 tion a b, (Plate I. fig. 6.) it would show the true position of the 

 arched strata : but if we suppose a section to be made only on the 

 side c d, an observer would see the face or escarpment on that side, 

 with the edges of the strata lying horizontally, and might describe 

 them as horizontally stratified, were he to view no other part of the 

 mountain. In some situations, the fracture made in the arched stra- 

 tification is much broken, and we have, on the side of the same 

 mountain, the appearance both of horizontal and greatly inclined stra- 

 tification. An instance of this occurs near the Lake of Bourget in 

 Savoy. Plate II. fig. 1. represents the appearance of strata on the 

 side of a mountain, which has the arched stratification before descri- 

 bed ; but the outermost strata, instead of enfolding the whole moun- 

 tain, only cover the southern side, and are broken off at the summit 

 in a line nearly parallel with it, and their edges present the appear- 

 ance of horizontal strata, a a. Lower down the mountain, part of 

 the under strata have fallen off in a sloping direction, and their pro- 

 jecting edges present, at a distance, the appearance of highly incli- 

 ned strata. This maybe further illustrated by taking a half cylinder, 

 -or, for want of that, a thick book, and opening it a httle ; place it 

 with the edges upon the table, and the back uppermost; cover the 

 book or half cyhnder with a number of folds of paper of different 

 colours, — these will represent arched strata. Cut across the outer- 

 most folds along the back, and take away the other half; the edges 

 of the paper will represent those of the upper strata, and their posi- 

 tion will appear to be horizontal. Cut away the corners of the un- 

 der sheets a little behind each other, so that the edges of each col- 

 oured sheet may be visible, and these will represent the appearance 

 of highly inclined strata, and they have frequently been mistaken for 

 such. The young geologist may greatly facilitate the study of stra- 

 tification, by laying coloured planes of any soft and yielding sub- 

 stance over each other, and inclining them in various positions ; then 

 let him make sections in different directions with a knife, and also 



