288 Correspondence — Prof. J. Le Conte — Mr. G. H. Klnahan. 



was justifiable in defending what were his own well-matured views, 

 as well as those of the surveyors, against an attack, which, however 

 learned, was apparently based upon work in the museum and library." 

 " Palseontological evidence is a powerful assistant to stratigraphy, 

 but it must yield precedence to results clearly made out in the field." 

 The Professor in his reply enumerates his extensive continental 

 labours and studies, thus justifying my estimate of his learning. 

 But it seems to me now, that his foreign studies have been carried 

 too far. For his comment on the above quoted passage is, that he 

 hopes I am the only geologist who will regard such action (the study 

 of foreign geology) as constituting a disqualification on his part — an 

 interpretation certainly "foreign" to my meaning. 



0. Fisher. 



EATE OF DENUDATION. 



Sir, — The appearance in your March Number of the letter of Mr. 

 McJames asking explanation of some points in Mr. Tylor's letter 

 induces me to notice some errors in the latter. 



1. Evidently by a slip of the pen Mr. Tylor has given v^ instead 

 of v^ as the law of variation of transporting power of running water, 

 although his calculations are based upon the true law : v^. Mr. 

 Tylor is right, therefore, in stating that, when the velocity increases 

 3 times, the transporting power is increased 729 times, and not 243 

 times, as Mr. McJames thinks it ought to be. This law was estab- 

 lished by Mr. Hopkins in 1842 (Phil. Mag. 1845, vol. xxvii. p. 56), 

 and is now universally accepted. It may be stated thus : " The 

 weigld of the largest fragment {of given form and sp. gr.) transportable 

 hy a current varies as the sixth power of the velocity." 



2. But transporting power must not be confounded with erosive 

 power. This is Mr. Tylor's mistake, and it vitiates all his calculations. 

 The resistance to be ovei'come in the one case is weight, in the other 

 cohesion. The one varies as v'^, the other probably v'^. In many cases 

 of lightly cohering material the resistance is a mixture of these two 

 resistances and the power of removal will vary somewhere between 

 «^ and v^. Joseph Le Conte. 



Berkeley, California. 



JUKES AND THE SUPPOSED LAUEENTIAN OF DONEGAL. 



Sir, — I have taken advantage of the first leisure I have had to 

 look up my notes for the " Geology of Ireland," and have to request 

 that you will publish the following. 



In 1862, after describing the Laurentians or primary gneiss of 

 Sutherland, Jukes goes on to say : — " Ireland — It is probable that 

 some of the highly metamorphosed rocks of the north of Ireland 

 may consist of this Pre-Cambrian gneiss." 



G. H. KiNAHAN. 



