364 WHITMAN CROSS 
presence of feldspars in some amount in all rocks, and also cer- 
tain characteristic associations of minerals. It indirectly recog- 
nizes structure and geological age as factors in classification. 
Roth believed that the classification by mineral components out- 
lined certain combinations or associations as central points of 
notably frequent and widespread occurrence, about which the 
rocks of other combinations and of comparative rarity must be 
grouped. While expressing the idea that not only the kind but 
the amount of various minerals might be considered in a mineral 
classification, he confined his own adaptation of this idea to the 
recognition of essential and accessory constituents, in certain Cases, 
and with no definite plan. 
Th. Scheerer, 1864.—Shortly after the appearance of Roth’s 
tables of rock analyses, a proposition for the general classification 
of igneous rocks upon a chemical basis was published by Th. 
Scheerer.* This proposition is notable as representing the views 
of one of the men who, in the preceding decade, had devoted 
much time and labor to the bulk analysis of rocks. Unlike 
Roth, who had come to the belief that no simple and persistent 
chemical formule or ratios could be set up for igneous rocks, 
Scheerer was convinced that he had found such formule, and 
that all silicate rocks could be referred to nine comparatively 
simple chemical types. 
Upon the basis of earlier investigations of the gneisses of 
the Saxon Erzgebirge, Scheerer had come to the idea that all 
of them could be considered as mixtures of three magmas, 
represented in nearly pure form in three prevalent gneisses of 
the region, namely: they “Rother Gneiss, 4 <Mittleres (Gneissias 
and ‘‘Grauer Gneiss,’’ the composition of which might be 
expressed in simple formule, given below. The visit to the 
Fassathal was made to test the general applicability of this three- 
fold division to other highly siliceous rocks, and to gather from 
the numerous basic rocks of that region material for chemical 
™“Vorlaufige Bericht itiber krystallinische Silikatgesteine des Fassathales und 
benachbarter Gegenden Siidtyrols,” Meues Jahrbuch fiir Mineralogie, etc., 1864, 
pp. 383-411. 
