158 
THE GEOLOGIST. 
Mr. Cox has laudably determined that the contents of the cave shall not be 
broken \ip nor disturbed, anticipating the probability of its being honoured with a 
royal visit. 
Polar Forces and Cleavage : Centres of Creation. — X. Y. Z. (Dublin). — 
" Sir, — If it would not be trespassing too much on your kindness, would you 
favour me with answers to the following, through the medium of your monthly 
paper The Geologist ? — It has always seemed to me that geologists, in speaking 
of polar forces, in reference to cleavage, only shift tho difficulty by giving it a 
name, as I have never been able to understand what they mean when iltey use this 
term, or how such forces could act effectually. Can you inform me under what 
plea any geologist objects to there being only one centre of creation, and 
requires more '? I cannot imagine, and have never been able to find anywhei'e 
stated, what the necessity may be which renders it necessary to suppose more 
than one. Lastly, perhaps you can suggest to me some elementary and, at the 
same time, extensive and sound work on the previous of these two questions, viz., 
cleavage." — The hypothetical application of polarity as an agent in the cause of 
cleavage is not admitted now by most geologists, lateral pressure being the 
fashionable doctrine. The well-known polar forces of electricity and magnetism 
have been used hypothetically and by way of analogy by some authors, in 
explanation of the changes which some of the old rock-masses have undergone, 
and in reference to the formation of certain mineral veins. This subject has been 
well treated by Mr. Robert Hunt, in the " Memoirs of the Geological Survey," 
and by Mr. R. W. Fox, in the " Philosophical Transactions," and there is a 
resume of the subject by the late Sir Henry de la Beche, in his " Geological 
Observer." Mr. Evan Hopkins has given a very wide range to the probable action 
of these so-called " polar forces." St. Helena, Galapagos Islands, Van Diemen's 
Land, Australia, and many other instances might be adduced of the existence of 
local faunas and floras, which it would be very difficult to understand if we were 
to suppose there had been only one centre of creation. This subject is well treated 
in Sir Charles Lyell's " Principles of Geology," and, with respect to the floras 
especially, has been well illustrated by the late Professor Edw. Forbes, in the 
" Memoirs of the Geological Survey." Our correspondent's last question facetiously 
reminds us of tho old woman with bad eyes, who went to the Boston bookseller for 
a new Bible of the largest print, smallest size, and cheapest price. The question 
is, however, a very proper one, and we reply to all such with much pleasure. 
Unfortunately there is no such collective work as X. Y. Z. requires on the subject 
of slaty cleavage. Mr. H. C. Sorby and the late Mr. Daniel Sharpo have written 
some admirable essays, and have done much in elucidation of this matter, and 
there i$ the first part of a report by Professor J. Phillips, in the " Report of the 
British Association for 1857," in which a general review of what has been done is 
commenced, and in which the chief labours hitherto made in this direction will be 
found embodied. The last edition of Lyell's " Manual " contains a resumi of 
some of the later researches on this point, and Mr. Jukes, in his " Manual of 
Geology," lately published, ably treats of it in his chapter on Physical Geology. 
Elementary Works. — J. H. M. (Staines) asks what is the best elementary 
geological work for a young man to study ? — Sir Charles Lyell's " Elements of 
Geology," and " The Principles of Geology," by the same author, "The Wonders 
of Geology " and "The Medals of Creation," by Dr. Mantell, are first-rate books. 
The last edition of the " Wonders " being printed under the revision of T. 
Rupert Jones, Esq., the assistant-secretary of the Geological Society, the newest 
discoveries and latest determination of the science are in it engrafted on the 
original labours of that late eminent geologist. Sir Charles Lyell, we all know, 
brings every new edition of his two celebrated works completely up to the mark. 
Other excellent works which occur to us are Page's "Text-book of Geology;" 
Professor Ansted's " Geology ; " Professor Phillip's and Professor Jukes's 
Manuals, and the late Sir Henry De la Beche's " Geological Observer." Miss 
Zornlin has written, too, some very pretty rudimentary books. 
Allopiiani;. — Z. (pnge 116.)— AUophane occurs at New Charlton, near 
Woolwich, Kent, in chalk, and is of a white honey yellow, or red colour. That 
