Washington — Catalan Volcanoes and their Rocks. 237 



therefore, the other 10 per cent of normative olivine has prob- 

 ably taken up silica to form modal augite, thus increasing the 

 amount of nephelite potentially present in the mode over that 

 shown by the norm, and this, with the feldspar, is represented 

 by the glass base. 



Chemical ^composition. — The chemical composition of the 

 rock of the massive flow exposed at the Fuente de San Roque 

 is shown in column VII of the table of analyses on page 239. 

 It does not differ in any essential respect from the others, the 

 only feature of note being the slightly higher ratio of potash 

 to soda, though the figures for each of these can be almost 

 duplicated from the other analyses, and the ratio is but 1 : 2, 

 so that the rock is clearly dosodic. Alumina is slightly higher 

 than in the other analyses, but ferrous oxide greatly surpasses 

 ferric, and the amount of titanic oxide is of the same order as 

 in the rest. 



Classification. — The norm of this rock is as follows : 



Or .'. . 1 3-34 ) _Sal_ _ Class III 



Ab... 11-79 U7-37 ) Fern" Salfemane 



An .. 22-24 ) )- 54-61 



Ne... 7-24 



Di... 17*79 



01 ... 14-76 



Ne... 7-24 7.24) F _- 6 . 54 Order 6 



Di... 17-79? ^ L Portugare 



32-55] 

 ! 



j 



f - Jl! i 11-08 45-31 *fl+W = 1-11 *»* 3 



11 — 7-60^ CaO Limburgase 



Ap... i*e 



Rest.. -46 J KO Subrans; 4 



Na O ~~ °' 50 Limburgose 



100-38 2 



The high ordinal ratio, which is near the limiting value of 

 7'00, indicates that the rock is close to the border of the 5th 

 order, gallare, and that it might be regarded as transitional 

 and called a camptonose-limburgose. But the simpler term 

 would seem to be sufficient for the present. In the prevailing 

 classifications the rock is undoubtedly to be considered as a 

 limburgite, since no feldspar is present, but only glass as a col- 

 orless (salic) constituent, along with augite, olivine and ores. 

 Limburgite is used here in the sense of Rosenbusch* as denot- 

 ing "the feldspar- free extremes of the rock series of the 

 trachydolerites, tephrites and basanites, and the leucite-, 

 nephelite- and melilite-rocks : they represent therefore the 

 pyroxenic rocks of the essexite family and belong to the thera- 

 litic magmas." It is evident from this that Rosenbusch 

 intends to denote by the term limburgite a rock derived from 

 magmas so deficient in silica, and at the same time so distinctly 

 alkalic in character, that lenads (leucite or nephelite) would 

 * Rosenbusch, Elemente der Gesteinslehre, 1901, p. 376. 



