146 PROCEEDINGS OF THE BOSTON MEETING 



absence of any noticeable evidence of dilves or necks of basic rocks traversing 

 the structures below the planes in which these Alpine green rocks are found 

 makes it difficult to consider them as flows that have been poured out from fis- 

 sures or vents near where they are now found. All these rocks, with the 

 exception of those of spilitic characters, are more or less free from any alka- 

 line characters. Even in the case of the nepheline-syenites of the Pilands- 

 berg. the former assumption of consanguinity between these and the rocks of 

 the Bushveld complex is weakened, though not completely disproved, by the 

 more recent studies. 



These are the types of eruptive rocks produced in periods of some regional 

 orogenic activity, of which the first series, the laccolitic, shows the least evi- 

 dence of lateral pressures. This leads to a fourth group, that of the great 

 dolerite-sills, such as those of the Karoo, in which there is little or no evidence 

 of crust movement accompanying the intrusion. In these sills gravitational 

 differentiation is sometimes in evidence, though rare, as in the case of the 

 Palisades on the Hudson and the laccolitic enlargements of the Karoo sills in 

 northeast Cape Colony. In such, rocks there is no evidence of alkaline char- 

 acters. In certain regions, particularly in the Western Isles of Scotland, ex- 

 tensive fissure eruptions of basaltic rocks which have very slight alkaline 

 characters (shown by the alkaline character of the zeolites and the occasional 

 presence of nepheline-bearing rocks and ouachitite) have been strongly folded 

 about centers of plutonic intrusion where a succession of strongly calcic 

 olivine-bearing rocks and gabbros with granophyre and granite have invaded 

 the basalts. These show no clear evidence of differentiation in situ gravita- 

 tionally, for the granite frequently lies beneath the basic rocks which it in- 

 vades. A regional series of intrusions of basalt dikes and dolerite sills into 

 the basalts closed the igneous activity. 



On the other hand, associations of this type lead us to more alkaline occur- 

 rences, of which several divisions may be recognized. The stable regions of 

 the earth's crust, which have not been subjected to such subsequent folding 

 (which are continental areas rather than geosynclinal), such as the Bohemian 

 Mittelgebirge and other plateau areas, have been the site of the development 

 of rocks of the essexitic-theralitic sequence, among which the picrites form a 

 series of basic sills. Not infrequently such extrusion was associated with 

 block-faulting, but not with folding. The rocks of the spilitic series, as defined 

 by Flett and Dewey, are also developed in areas in which there was an absence 

 of folding forces during the period of eruption. They occurred, however, in 

 geosynclinal areas that were undergoing steady subsidence in the period prior 

 to an orogenic movement. Studies of thick albitized sills and of the large 

 masses of associated albitic rocks (keratophyres) made by the writer and 

 others do not support the hypothesis that their features result from the action 

 of resurgent or even juvenile water, concentrating the alkalies in the upper 

 portion of the magmas, when in their present position, but indicate a com- 

 130sition for the immediate parent magma of spilites and keratophyre more 

 sodic than that of normal basalts, though the possible effectiveness of resur- 

 gent water in adding to the pneumatolytic activity of such intrusions is not 

 excluded. In the development of rocks of this group shallow intrusions of 



