594 PETROLOGY AND MINERALOGY 



rare substances as vanadium, barium, and strontium are present in 

 nearly all the eruptive rocks, and that even molybdenum occurs with 

 surprising frequency, although in very small quantities. In this con- 

 nection W. F. Hillebrand is especially deserving of mention, his prac- 

 tical Guide to the Analysis of the Silicate Rocks being a perfect 

 treasure-house of experience and practical hints. He very properly 

 insists on the desirability of cooperation between the chemical and the 

 microscopical study of rocks, now so commonly kept separate, and 

 points out that if the examination of the thin section always preceded 

 the chemical analysis the latter could be carried out with greater ease 

 and exactness. 



The literature of chemical petrography has recently been enriched 

 by a truly monumental work, also executed with wonderful industry 

 on this side the ocean. Henry Washington, following in the footsteps 

 of Justus Roth, but with a more modern point of view, has succeeded 

 in assembling and critically reviewing all the analyses of eruptive rocks 

 and tufas which have been published during the sixteen years from 

 1884 to 1900. Beside the introductory remarks dealing with the selec- 

 tion of material, the amount of material, the measure of the degree of 

 accuracy and completeness of the analyses, the sources of error, et 

 cetera, the work is of primary importance as being the first attempt 

 to appraise justly and impartially the relative values of the analyses. 

 Adopting a method similar to that used in estimating the credit of a 

 commercial business, he undertakes to arrange these analyses, accord- 

 ing to their degree of accuracy and completeness, in five groups, de- 

 signated in descending order as "excellent," "good." "fair," "poor," 

 '''bad." He has made a beginning most deserving of acknowledg- 

 ment, and it is to be hoped that it will serve as a warning cry to ana- 

 lytical chemists. 



In order to determine the composition of a rock-species the satis- 

 factory chemico-petrographic analysis must show both the percent- 

 ages of the various component materials as well as afford an insight 

 into the position the rock occupies in certain chemical series. While 

 a normal series is characterized by a steady increase and decrease in 

 the materials, the peculiar ultra-members are especially noteworthy 

 in this respect. Thus we have the great independent group of the 

 Eruptives, which in spite of great basicity is almost wholly lacking 

 in alumina and alkalies, although enormously rich in magnesia. 

 Perhaps an even more striking case is that of a rock containing 

 scarcely twenty per cent Si0 2 , with almost all the remainder of its 

 composition consisting of A1 2 3 , and yet the rock is a true Intrusive. 



In recent years there have been many attempts to express the 

 relationships of rocks by using simple formulated expressions for the 

 chemical rock-composition. There have been also endeavors to show 

 the relative position of an analysis by graphic methods which bring 



