ROSS — ON PARALLEL LINES OF ELEVATION. 129 



to view. Nor are we always left to infer the lines of elevation of 

 the older rocks, from those portions of them which are exposed to 

 view, since as we have already seen they impress their lines 

 of elevation, to some extent upon the superincumbent later 

 formations. 



By an attentive study of the lines of elevation in the earth's 

 crust, in so far as I had the means of information, I find that all 

 the more important of them group themselves into seven great 

 zones of parallel lines of elevation; the axial line of each zone 

 being approximately a great circle of the earth, and characterized 

 by lines of volcanoes. The apparent development of these zones 

 is much affected by the fact that the northern hemisphere has the 

 surface of its solid crust considerably more elevated than is that of the 

 southern hemisphere. At the 45th degree of latitude respectively the 

 difference by a rough estimate is nearly two miles. But if, for the 

 sake of illustration, we imagine the north pole of the earth in 

 Iceland, and compare the elevation of the resulting northern and 

 southern hemispheres at the thirtieth degree of latitude respectively, 

 the average elevation would then be found to be more than two miles 

 greater in the northern than in the southern. Hence result the 

 following laws : — 



1st. — That it is the half north of the equator of each zone that is 

 chiefly exposed to our observation. 



2nd. — That of those zones whose axial lines do not pass near 

 the poles, it is on the northerly side of the northern half of the 

 axial line that the chief visible development occurs. 



In each zone the proximity and elevation of the anticlinals diminish 

 gradually from the axial line outwards ; and if zone No. 1 be con- 

 sidered the most recent, and the others as successively less recent in 

 the order in which I have named them, and comparing similar parts of 

 any two zones, the height of the anticlinals.is greater, the dip less, and 

 the distance between their axes greater in the more recent. In a 

 transverse section of zone No. 1 or of any subzone of any of the 

 other zones, each plateau rises above the preceding in regular 

 gradation from the coast line (or other boundary of the zone or 

 subzonej until the greatest elevation is reached ; and the same is 

 true of the mountain ridges separating the plateaus. By substitut- 



