260 GEOLOGY. 



the average dip of the beds is taken and the distance across the trun- 

 cated edges, EF, is measured and the thickness of the whole computed 

 by trigonometry. The angle at G being a right angle, the angle FEG and 

 the side EF being known, the length of FG is readily ascertained, and 

 this is the thickness of the series, as usually measured when the thick- 

 ness is great. By comparing the line GF, representing the thickness 

 of the series, with the line CD, the depth of the ocean, a marked difference 

 will be seen. Moreover, consideration will show that the difference 

 may vary indefinitely, and that there is no necessary relation between 

 the two. If the line EF be short, as it would be if the series had not 

 been built far out, the thickness FG would be less than the depth of the 

 ocean, CD ; but if the line EF be long, as it would be if deposition were 

 continued sufficiently, the thickness of the series, FG, would be pro- 

 portionately increased, while CD might remain constant. In other 

 words, the thickness of the series may vary from any fraction of the depth 

 of the basin to any multiple of it, within certain limits imposed by the 

 width of the basin. 



Similar considerations reveal a discrepancy between the vertical 

 measurement and the thickness of the beds deposited subaerially. For 

 example, the thickness of the beds of a typical volcanic cone is less 

 than the height of the cone which they form, as will be seen from Fig. 109. 

 Lava-flows poured forth on heights, and congealing as they spread out 

 upon the slopes below, may give rise to series of great aggregate thick- 

 ness, without implying any contemporaneous sinking or other crustal 

 change. Clastic beds formed by slope-wash may become interleaved 

 between these lava-flows without implying subsidence. In this case, 

 the horizon of wash is shifted to higher and higher positions in the series 

 (not necessarily to higher altitudes), but the series of beds is not lowered. 

 In the case of the Keweenawan system, a congelation or deposition 

 slope of 5°, extended horizontally a little over one hundred miles, or 

 about half across the Keweenawan basin, as it may have been before 

 compression, would give the estimated thickness (see p. 192). To 

 explain the present attitude, it is necessary to suppose that the series 

 was compressed laterally, so that the beds were upturned and somewhat 

 sheared upon one another (Fig. 110). A greater upturning must of 

 course be supposed, if the alternative view is taken that the beds 

 were originally strictly horizontal. 



It is not here affirmed that this is the explanation of the great ap- 



