376 HENRY S. WASHINGTON 



uniformly leucitic, while those with silica above 60 are as uniformly 

 leucite free and orthoclase bearing. It is in the intermediate 

 group with silica from 55 to 60 that we must look for evidences 

 of the connection of leucite with extrusive conditions. These 

 magmas are apparently in a nicely balanced chemical condition, 

 which needs only a comparatively slight change of conditions to 

 throw them into the one mineralogical group or the other. It 

 is known* that the leucitic rocks are almost invariably met with 

 as flows, while the trachydolerites more frequently take the form 

 of domal extrusions ; that the leucitic rocks are the products of 

 later eruptions, on the whole,. than the trachydolerites except at 

 Rocca Monfina ; and that they have been ejected at central vents 

 while the trachydolerites are more generally peripheral. This 

 is suggestive of the idea that leucite has been formed in place 

 of orthoclase during the simmering of the magma in the throat 

 of the volcano under low pressure,' while the peripheral ortho- 

 clase magmas were partially crystallized under greater pressure. 



In view of the above facts and those given in the pre- 

 ceding section I am inclined to think that the two main groups 

 of rocks which are found along the Bolsena- Vesuvius line do 

 not represent two distinct primary differentiation products, but 

 that their differences are due to diverse conditions of extrusion, 

 solidification, and the like, these conditions being, of course, 

 secondary to the chemical character of the differentiation prod- 

 ucts, as has just been explained. 



In regard to the character of the parent magma we may feel 

 confident that it was very rich in potash and lime,=' basing our 

 judgment on the preceding descriptions and analyses. More 

 than this we cannot postulate with any degree of certainty, 

 but its general composition is perhaps shown by the mean of 



'Cf. Backstrom, op. cit., p. 163; also Lacroix, op. cit., 637 ff. 



= The hypothesis advanced by Lavis (Natural Science, IV, 138, 1894) that the 

 large amount of lime is due to absorption (osmosis) from-the subjacent limestone 

 traversed by the magma must be mentioned, though the writer can only admit its 

 influence in a most limited way, if at all. The researches of Brogger and Pirsson 

 have shown conclusively that it does not hold good elsewhere, and various considera- 

 tions, which need not be gone into here, lead me to minimize its effect in Italy also. 



