IO WM. H. HOBBS 
given rock type must know the range in the percentages of the 
eight principal constituents. Moreover, he is not assisted in this 
by the knowledge that the upper and lower limits which he learns 
for a constituent of one species are at the same time, respect- 
ively, the lower and the upper limits for the same constituent in 
other allied species. A tax is thus imposed upon the memory far 
beyond what it may be reasonably expected to bear, and this tax 
is increased with the fixing of each new rock species. 
The eye assists the mind not only to discover intricate rela- 
tionships, but also to retain them, whenever the facts can be 
expressed by a definite form. This has been appreciated espe- 
cially by the engineering profession, which has been accustomed, 
by the use of diagrams, to set forth in the most lucid manner 
facts which only the most laborious methods could otherwise 
bring out of the tables on which they are based. A curve con- 
tains the essence of pages of figures, and is readily carried in 
the mind owing to the large development of that faculty, which 
the Germans have so aptly termed Vorschauungsgabe. It is note- 
worthy that so little attempt has been made to apply graphic 
methods in petrology. 
Recently, however, Iddings,* Becke,? Michel-Lévy? and 
Brogger+ have each devised diagrams to illustrate rock analyses. 
Of these the diagrams of Brégger seem to me the ones best 
adapted for general use because the simplest and the most char- 
acteristic. In the Bréggers diagram are set off on radius vectors 
the amounts of the eight principal chemical constituents reckoned 
tJ. P. Ippines: The Origin of the Igneous Rocks, Phil. Soc. of Washington. 
XII, pp. 89-214. Pl. II, 1892; Absarokite-shoshonite-banakite series. JOUR. GEOL., 
III, pp. 90-97, 1895; On Rock Classification, zézd., VI, p. 92, 1898; Chemical and 
Mineralogical Relationships in Igneous Rocks, 2d2d., p. 219. 
2F. BECKE: Die Gesteine der Columbretes. TSCHERMAK’S min. u. petrog. 
Mittheil., VI, p. 315. 1897. 
3M. MicHEeL-LEvy: Porphyr bleu de l’esterel, Bull. de la service de la carte géol. 
de la France. TomeIX, No. 57, 1897; Sur une nouveau mode de co-ordination des 
diagrammes representant les magmas des roches éruptifes. Bull. de la soc. géol. de la 
France. (3) XXVI, p. 311. 
4W.C.BrOcGER: Die Eruptivgesteine des Kristianiagebietes; Das Ganggefolge 
des Laurdalits. Kristiania, 1898, p. 255. Pl. I. 
5Or LEVY-BROGGER diagram. 
