Sir H. H. Howorth — Boulders on the Yorkshire Coast. 371 



in defining the several zones; for the more perfect the forros and 

 the higher the development, the less likely are they to have a great 

 range." 



Though Trilobites were found by us to be of greatest value in 

 defining and correlating the zones in the Cambrian rocks, in 

 succeeding sediments Graptolites and other organisms have proved 

 of equal value. Sometimes it so happens that the forms which 

 appear to be the most typical of an horizon in one area are entirely 

 absent from another geographically not far distant ; but in such a 

 case, unless it can be shown that the physical conditions must have 

 varied sufficiently to have affected the whole fauna, there are usually 

 evidences of a sufficient number of the associated organisms present 

 to enable the zone to be easily defined. 



In working out zones it is necessary, therefore, to register with 

 care all the forms which occur with the so-called zonal fossils, and 

 also the evidences in the strata of the physical conditions prevailing 

 when the sediments were accumulating. 



[To be continued.) 



VII. — The Primitive Boulders of the Yorkshire Coast and 

 THEIR Lessons. A Eeply to Two Critics. 



By Sir Henry H. Howorth, K.C.I.E., M.P., F.R.S., F.G.S. 



I CANNOT sufficiently welcome the two letters devoted to myself 

 in the July Number of the Geological Magazine; and I only 

 hope Mr. Harker and Mr. Deeley will continue to face the issues 

 between us, and not be content, as others have been, with fulminating 

 more or less testy protests, and then retiring from the field. 



Mr. Deeley objects to my statement that the tendency of the 

 most philosophical writers on geology is to go back from the 

 scholastic formula of Uniformity enunciated by Lyell and Eamsay 

 to more inductive reasoning. With the writings of Suess and 

 Lapparent, of Spencer and of Prestwich, before me, all of them 

 veterans of the first class, I cannot either understand or appreciate 

 Mr. Deeley's protest. Perhaps he means that the geological world 

 is limited to English official geologists ; most of them scholars of 

 Jukes and of Kamsay, who naturally echo their masters' views. 

 He will not expect me to agree with this delusion. 



Turning from this wide issue to the narrower one of ice-sheets, 

 I must again emphatically say that English geology is making 

 itself ridiculous in the eyes of men trained as mathematicians and 

 physicists by some of its professors habitually appealing to causes 

 and to dynamical forces whose competence has not been proved 

 or even shown to be consistent with the laws of matter. That ice 

 moves as a plastic body has been abundantly proved by experiment. 

 That moving thus it can travel over hundreds of miles of level 

 country, or up and down great hollows like the Baltic and the 

 North Sea, without an adequate vis a tergo, is incomprehensible to me. 

 That such a vis a tergo can only, in the case of such ice-sheets, be 



