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are called volcanic blocks. Geikie defines lapilli as ejected fragments of lava of various shapes varying in size from a pea to a walnut. He calls all the fragments smaller than lapilli volcanic sand or volcanic ash. His definitions of fragmental volcanic rocks are essentially the same as those of Pirsson.
These writers agree in using size as the basis for classi fication of pyroclastic rocks but the above definitions are not satisfactory, because nuts, apples, peas, and shot vary considerably in size. More definite size limits together with a statement of the proportions of various sized particles in a given pyroclastic rock are needed before any uniformity in the usage of these terms can be attained. Thus the same rock may be called volcanic breccia or agglomerate by one writer and volcanic ash or tuff by another. It is the tend ency of many geologists to call any pyroclastic rock or epiclastic rock containing much volcanic material, "volcanic ash," regardless of what its texture may be.
For the sake of clarity in the discussion of pyroclastic rocks and similar rocks which have been formed by the erosion and redeposition of pyroclasts the writer proposes the classification given below. This system is adapted from Udden's classification of clastic sediments. On account of the very poor sorting in most pyroclastic rocks only a few of Udden's subdivisions 50 are used. Wentworth's system sl of class terms for clastic sediments has also been introduced with considerable modification and simplification into the scheme proposed for pyroclastics. The writer prefers to use 50% as the class division point for pyroclastics, instead of the 80% division point used by Wentworth for sediments, because clastic volcanic rocks are rarely as well sorted or graded as many sediments are.
50 Udden, J.A., "The Mechanical Composition of Clastic Sediments," Bull. G.S.A., Vol. XXV, pp. 655-744, 1914.
siWentworth, C. X., "A Scale of Grade and Class Terms for Clastic Sediments," Jour. Geol., Vol. 30, No. 5, pp. 377-392, 1922.
