Some Recent American Petrological Literature. 179 



olivine, -with little felspar : at five of them are late trachytic and 

 phonolitic lavas, while two show coarsely crystalline cores of gabhro 

 or theralite and in one case syenite and nepheline-syenite. 



"Age of the Igneous Rocks of the Adirondack Region," by A. P. 

 Cushing. Amer. Journ. Sci., vol. xxxix, pp. 288-94, 1915. 

 The orthogneisses which have been metamorphosed along with the 

 Grenville rocks have been invaded by the anorthosite-syenite group, 

 which contain inclusions of the gneisses. It is held that the 

 orthogneisses and the anorthosite-syenite group are distinct and 

 should not be classed together. 



"The Composition of the Average Igneous Rock," by A. Knopf. 

 Journ. Geol., vol. xxiv, pp. 620-2, 1916. 

 Using the data given in Daly's Igneous Hocks and their Origins, the 

 writer calculates a new average igneous rock, taking into account 

 the actual volume of each type, so far as known. This average is 

 found to agree very closely with that previously given by Clarke. 



" The Summation of Chemical Analyses of Igneous Rocks," by H. H. 

 Robinson. Amer. Journ. Sci., vol. xli, pp. 257-75, 1916. 

 An application of the laws of probability to the determination of 

 errors of summation, as a guide to the quality of the analytical work. 



"Use of the Slide-Rule in the Computation of Rock Analyses," by 

 J. H. Hance. Journ. Geol., vol. xxiii, pp. 560-8, 1915. 

 The author gives two tables of the percentage composition of rock- 

 forming and ore minerals with a method of use applicable to an 

 ordinary slide-rule for the conversion of analyses into the 

 corresponding mineral compositions. 



"Suggestions for a Quantitative Mineralogical Classification of 

 Igneous Rocks," by A. Johansen. Journ. Geol., vol. xxv, 

 pp. 63-97, 1917. 

 The basis of the proposed classification is a figure in the form of 

 a double tetrahedron, each corner representing certain mineral 

 constituents of the rock, the composition of any type being repre- 

 sented graphically in its proper position in the figure. The minerals 

 are divided into groups and a large number of new rock-names are 

 proposed. 



"Types of Prismatic Structure in Igneous Rocks," by R. B. 

 Sosman. Journ. Geol., vol. xxiv, pp. 215-34, 1916. 

 Several types of prismatic structure are distinguished as due to 

 thermal contraction, convection in the still liquid magma, and 

 internal expansion respectively. 



"The Microscopical Characters of Volcanic Tuffs, a Study for 

 Students," bv L. V. Pirsson. Amer. Journ. Sci., vol. xl, 

 pp. 191-211, 191 . 

 A discussion of the origin and characteristic structures of tuffs, 



with a description of the criteria for their recognition when altered, 



weathered, and metamorphosed. 



