334 



REPORT OF NATIONAL MUSEUM, 1904. 



below to show the general character of the work and the now seeming 

 crudity of the illustrations." 



STRATIFICATION. 



35 



the strata appear to rest against the sides of the mountain 

 masses; d being the moun- 

 tain, and a a, b b, c c, similar 

 strata of rocks. 

 d \\\\ There is another instance in 

 . which the strata may not ter- 

 minate at their outcrop, but 

 appear again in the opposite hill, as a a, but b b, by its 

 prolongation, would pass over the hill C. Where the 



strata are nearly hori- 

 zontal similar strata 

 are almost al ways found 

 on the opposite r sides of 

 6 a valley, as if they 

 had been formerly con- 

 tinuous, and the valley 

 since formed by some powerful cause. 



There is another point in relation to stratification, (and 

 it is also applicable to veins, in rocks that are not stratifi- 

 ed,) that, (rom its practical importance in mining, should 

 be well understood. The bed of coal, or vein of ore, ap- 

 pears suddenly to termin- 

 ate. When this occurs, the 

 bed may be found again, 

 either above or below its o- 

 riginal level. At its appa- 

 rent termination, a fissure, 

 or else a different kind of 

 rock, occurs, generally in the form of a vein, as c c. Re- 

 peated observation has shown, that if. at the apparent 

 termination, the rock cutting it off inclines towards you, 

 so as to project over your head, the bed of coal, or other 

 mineral, lies at a lower level on the other side of c c, as 

 though the mass of strata on one side of the vein had slid 

 down to a lower level. In the figure, a a and b b repre- 



Does the same stratum ever appear on the opposite 

 sides of vallies ? When beds of coal terminate sudden- 

 ly, can they be regained ? and how ? What has been 

 observed of ihe positions of the beds of coal, in refer- 

 ence to the sloping of the vein or dyke? 



Fig. 27. — Page from Mather's Elements of Geology. 



" A copy of this book, in the possession of the present writer, has the following 

 printed indorsement pasted on the inside of the front cover: 



[Recommendation of Professor Silliman, of Yale College.] 



Dear Sir: You ask my opinion of the Elements of Geology, for the use of schools, 

 by Wm. W. Mather. I think that it is a judicious, correct, and perspicuous work — 

 containing, in a small compass, a solution of many of the most important facts and 

 theoretical views in geology, and that it is well adapted to the object for which it 

 was written. 



Yours, respect fully, 



B. Si ill man. 



Mr. Wm. Lester, Jr. 



New Haven, June 18', 1834- 



