288 Correspondence — Prof. Joseph Le Conte. 



whatever movement of shore-line there was, seems to have been 

 westward with increase of land. 



Two other points I briefly touch. Prof. Hull thinks that I do not 

 recognize sufficiently, if at all, his most important point, viz. " the 

 increase of thickness of sediments to the N.E. and E., and their 

 attenuation and replacement by limestone in the opposite direction." 

 If I did not lay stress on this, it was only because I supposed it 

 generally recognized, although Prof. Hull brings it out in a very 

 striking way in his figures. No one has emphasized these facts, and 

 their significance as showing a large land-mass to the north-east and 

 a wide ocean to the south-west, more than I have. 1 



Again, in my previous communication 2 I said, " There is no reason 

 why the eastern land-mass, which sufficed to contribute 30,000 ft. of 

 Silurian and Devonian sediments, should not have been sufficient to 

 contribute the much smaller amount of Carboniferous sediments." 

 Prof. Hull thinks this a begging of the question at issue. For, says 

 he, " the narrow strip of land allowed by Prof. Le Conte was quite 

 insufficient to produce 30,000 ft. of conformable sediments." I can 

 only say in reply that Prof. Hull's map of Silurian times led me 

 astray : for this shows just such a land-mass as I suppose, while his 

 map of Carboniferous times shows a very much greater land-mass. 

 I suppose, now, howevei", that he imagines this land-mass to have 

 increased on its eastern side through Silurian and Devonian times. 

 If so, it must have increased very rapidly, for the Silurian alone is 

 20,000 ft. thick in the Appalachian region. Joseph Le Conte. 



PERMANENCE OF CONTINENTS & OCEAN-BASINS, WITH SPECIAL 

 KEFERENCE TO THE FORMATION AND DEVELOPMENT OF 

 THE NORTH AMERICAN CONTINENT. 3 



g IR) — Will you allow me to make a correction ? Prof. Chamberlin 

 has kindly drawn my attention to the fact that in my original com- 

 munication to you on this subject 3 I have misrepresented him, and I 

 wish therefore to acknowledge my error. The map on p. 62 of 

 Prof. Chamberlin's work on the Geology of Winconsin was not 

 intended, as I supposed, as a map of Archasan areas, but really 

 as a map of land during a portion of Archaean times, viz. (if I 

 understand him) at the beginning of the period of Huronian 

 sedimentation. 1 was misled by its great resemblance to the usually 

 recognized map of Archaean areas. The confusion of thought to 

 which I referred does indeed exist, but Prof. Chamberlin is not an 

 example of it. 



Let us hope that Prof. Chamberlin will give us more fully his 

 mature views on this so obscure and yet so important subject. No 

 one is more competent than he to write with authority on the 

 subject. Joseph Le Conte. 



Berkeley, California, May 3, 1886. 



1 Am. Jour. vol. iv. p. 463, 1872. Elements of Geol. p. 289. 



2 See Geol. Mag. March, 1886, p. 100. 



3 Geol. Mag. 1886, Dec. III. Vol. III. p. 97. 



J 



