Reviews — E. A. Newell Arber — The Olossopteris Flora. 135 



averages serve to find the group to which any other igneous rock 

 belongs. The following two examples will illustrate this : — 



No. 1, a specimen of Elvan from Cornwall, recorded in Cole's 

 handbook. 



No. 2, a rock named Teschenite from Cape Verde, recorded by 

 H. S. Washington, p. 352. 



No. 1 .. 

 No. 2 .. 



First calculate the four values I, II, III, IV, which represent 

 the total of the seven bases (or four sets of bases) minus respectively 

 AI2O3, Fe^Os + FeO, MgO + Ca 0, Nas + K2O. The values 

 are as follows : — 



Following the scheme, p. 134, we find that sum I of example 

 No. 1 is below the average of 24'8, and is therefore to be marked 

 as (a). For comparison with 11 the reader must now take the left- 

 hand value of II = 28-4, and this gives again (a). For comparison 

 with III we have thus again the left-hand value III = 24'2. We 

 now have an excess and letter (6). The fourth letter is again (a), 

 because 18-7 is smaller than 22-6. The entire formula for rock 

 No. 1 is therefore {a a b a), group 3 (granite) of separate Table I. 



Sample No. 2, compared in the same way with the scheme, p. 134, 

 is found in all four cases to exceed the corresponding numbers 

 of the scheme; the formula is therefore (6 6 6 6), group 16 of 

 Table I. 



I^ IB "V I IE "W s. 



I. — Catalogue of the Fossil Plants of the Glossopteris 

 Flora in the Department of Geology, British Museum 

 (Natural History) ; being a monograph of the Permo- 

 Oarboniferous Flora of India and the Southern Hemi- 

 sphere. By E. A. Newell Arber, M.A., F.L.S., F.G.S., Trinity 

 College, Cambridge, University Demonstrator in Palseobotany. 

 (London, 1905.) 



THE present Keeper of the Geological Department of the British 

 Museum has continued the wise policy of his predecessor in 

 encouraging the production of comprehensive accounts of the fossil 

 floras represented in the National Collection. Mr. Arber's volume 

 supplies geologists and botanists with an excellent summary of the 

 present state of our knowledge in regard to the Glossopteris Flora, 

 and includes concise descriptions of the abundant material in the 

 Fossil Plant Gallery. The excellent illustrations by Miss G. M. 

 Woodward add considerably to the value of the work. 



It is a striking fact that the six volumes dealing with Fossil 

 Plants in the British Museum which have so far been published are 



