Correspondence — F. Dixey. 233 



Bagshot series. A plate is given of the type-section in Sim's Clay- 

 pit at Claygate. 



Considerable space is devoted to a description of tlie river terraces 

 and flood plain gravels, and a tentative correlation is given between 

 these deposits and the inaplement classification. 



A special chapter at the end deals with economic deposits, the 

 nature of the soils being described under each stratigraphical division. 

 An excellent bibliography is provided. 



The plates are very interesting, being reproductions of old prints 

 of London with notes on the geological j^oints they illustrate. 



Taken all round, the memoir is a great boon to geologists liviiog 

 in the area, both as a reference book and as a most lucid guide. 



J. E. A. W. 



The Oil Encyclopedia. By Marcel Mitzakis. pp. 551. 



London : Chapman & Hall, Ltd. 1922. Price 21s. 

 nrmS book is not only what its title indicates, a dictionary of 

 -*- all practical matters concerned with the oil industry, but it is 

 also something more, for it includes under the appropriate headings 

 short summaries of the information available on many points of 

 scientific interest, such as theories of oil-formation, the geology 

 of oil-bearing regions, the chemistry of oil and its products, and many 

 cognate matters. While in the compass of the short articles in this 

 volume it is impossible to go deeply into such subjects, nevertheless, 

 these summaries should prove extremely useful, while as a book of 

 reference for locating little-known and out-of-the-way oil occurrences 

 it will have great value. Short biographical notes are also given of 

 the principal workers in this branch of industry, both scientific and 

 economic. Where desirable references are also given to original 

 publications. 



CORRESPONDENCE. 



THE MAGNESIAN GROUP OF IGNEOUS ROCKS. 



Sir, — I would be glad if you would kindly afford me the 

 opportunity of replying to the points raised by Mr. Tilley in 

 the letter which appears in your issue of this Magazine of 

 December last. 



While Mr. Tilley acknowledges the existence of the group of rocks 

 I endeavoured to describe, he objects to the use of the term 

 " magnesian " on the assumption that the rocks under discussion 

 contain notable amounts of magnesium only in the ultrabasic 

 members ; moreover, he states that it is iron and not magnesium 

 that calls for attention in the less basic types. I have myself already 

 mentioned the importance of the iron in my paper. As regards the 

 magnesium, it is sufficiently clear, from the works of Holland, 

 Lacroix, and other writers I referred to, that the high content of 

 this mineral is far from being confined only to the ultrabasic members. 



