Correspondence — Prof. Joseph Le Conte. 



287 



the Dead Sea and of this valley are covered by alluvial deposits. 

 What the thickness of these may be no one knows ; nor can the 

 question of the depth to the solid rocks below the alluvial materials 

 be solved except by extensive boring operations. In my opinion the 

 depth is very great ; and if this be so, the answer to the question of 

 the reviewer is plain, — at least, this is the only answer I conceive 

 possible. 



Dtjnfannaghy, 12th May, 1886. EDWARD Hull. 



THE DEVELOPMENT OF THE NORTH AMERICAN CONTINENT. 



Sir, — There are no questions in Geology more important and more 

 fascinating than those of Palaso-geography. All geologists must be 

 grateful to Prof. Hull for the light he has shed upon them. But, 

 also, there are no questions which are more difficult, and the solution 

 of which is more illusory. I would not again trouble you on this 

 subject except to correct what seems to me a grave misconception on 

 the part of Prof. Hull, 1 which lies at the basis of nearly all the 

 difference between us. 



He refers to an ideal section of the Palaeozoic rocks on p. 288 of 

 my " Elements of Geology " (being a section from Canada through 

 New York to Pennsylvania), as indicating continued subsidence of 

 sea-bottom and retreat of shore-line northward during the Palaeozoic 

 period. This interpretation is the very opposite of that usually given 

 by American geologists. Perhaps the mistake, if it be one, is partly 

 due to bad drawing. In order to bring all the Palaeozoic strata 

 within the compass of a small figure, the southward dip is enormously 

 exaggerated. In fact, the strata are nearly level, the average dip 

 being probably not more than 15-20 feet per mile. The successive 

 appearance of younger and younger rocks as we go southward is 

 supposed by all American geologists to indicate a gradual elevation 

 of the Canadian land-mass of that time, and a consequent advance of 

 the shore-line southward with steady increase of land. This is seen at 

 once if the section be drawn with smaller dips and leaving out 

 details (Fig. 1). 



e d e_ h J*^<^§§. 



Fig. 1.— Generalized N.E. and S "W. section from Canada through New York to 

 Pennsylvania. A. Archsean. P. Primordial. L.S. Lower Silurian. 

 U.S. Upper Silurian. D.Devonian. £.6". Sub-Carhoniferous. abode 

 successive shore-lines. 1 1' I" V" I"" successive sea-levels. 



The western shore-line of the eastern land-mass was, on the con- 

 trary, nearly stationary, and hence the prodigious thickness of Palaeo- 

 zoic sediments in the Appalachian region. Even here, however, 

 1 See Prof. Hull's letter, Geol. Mag. April, 1886, p. 189. 



