TITLES AND ABSTRACTS OF PAPERS -147 



picrite are frequently present, and sometimes, as, for example, in tiie Devonian 

 igneous series in northwest Germany, and perhaps in tlie fourcliite of Ar- 

 kansas, the characters of the spilitic and essexitic-theralitic divisions of alka- 

 line rocks are somewhat intermiuijled. In general, however, these rocks, 

 formed in geosynclines, have been subjected to subsequent folding and are 

 greatly disturbed. The history of the region subse<iuent to the development 

 of the igneous rocks, by showing the nature of the crust conditions in the 

 region, geosynclinal or continental, is permissible in some degree as evidence 

 of the tectonic conditions at the period of the intrusion of the rocks. 



A final group is the very uniform series of mica-peridotite dikes of alnoitic 

 characters that occur in the regions west of the Appalachian Mountains and 

 in the Indian coal fields, and are represented by melilite-basalts in South 

 Africa. These are probably a very special group of dikes forming under con- 

 ditions allied to those which accompanied the formation of essexitic-theralitic 

 or foyaitic rocks (with which they seem to be associated in Arkansas) — that 

 is, they were formed in practically unfolded regions, with perhaps fracturing 

 and faulting of the crust, but not tangential pressure. 



This general survey leads to the conclusion that those later processes giving 

 rise to the evolution of igneous rocks have in all probability a much less im- 

 portant effect in those solidified magma basins, which are now exposed by 

 erosion, than they had in the deeper subcrustal reservoirs, in which the 

 I)hysico-chemical processes described by Smyth and Bowen, together with 

 some gravitational differentiation emphasized by Daly, may have been very 

 effective ; but the writer concurs with Dr. Harker in urging the effectiveness 

 of lateral pressure in the separations of the magma, the portions richer in alka- 

 lies, and generally those portions which under Bowen's scheme would form 

 the later consolidations from the magma, being moved toward the direction of 

 least lateral pressure. 



In regions where there is no great lateral pressure over broad areas the 

 Plutonic rocks are generally the normal sub-alkaline series of gabbro-granitic 

 rocks, but where lateral pressures are pronounced, there is a tendency for the 

 region where the pressure is least to become enriched in alkalies. The total 

 volume of alkaline rocks, as Professor Daly has shown, is so small in com- 

 parison with the rocks that do not show alkaline features that it is not possible 

 to distinguish with any certainty the products of a normal gabbro magma from 

 those of a magma rather impoverished in alkalines which have been strained 

 off toward areas of local enrichment in alkalies. Hence the gabbros and 

 granites of the laccolitic complexes are indistinguishable chemically from 

 those of the Cordilleran and Alpine types. This straining of the magma in 

 response to lateral pressure, leading to lateral differentation, may be either 

 of a regional or of a local character. In the former case, the characteristics 

 of the petrographic provinces produced may be more or less clear, but in the 

 latter the differentiation may be incomplete and lead to no great geographical 

 separation as a whole, though locally specialized types may occur, thus causing 

 development of tliose complexes of associated alkaline and sub-alkaline rocks 

 in which the affinities of the complex and of some of its members can not be 

 sharply defined into one or other of the two groups which, in Dr. Harker's 



