60 ANNALS NEW YORK ACADEMY OF SCIENCES 



of a sediment by an igneous rock is always of abnormal composition and 

 constricted extent and is usually marked by such minerals as sillimanite, 

 cordierite and wollastonite. By variation diagrams lie proves that such a 

 commingling would give rise to rocks whose composition might be pre- 

 dicted and which would not even approximate any known igneous flow. 

 In the Cortlandt Series, we have this latter type well developed in the 

 instances described; whether a normal igneous rock, such as syenite and 

 gabbro, may be formed, despite Harker's arguments, is a question requir- 

 ing more data. The rocks in question must be analyzed, and the variation 

 diagrams constructed for them and for the surrounding magmas, before 

 anything can be definitely said. It is interesting in this connection, how- 

 ever, to recall Daly's 46 theory of overhead stoping, which postulates exten- 

 sive assimilation of the blocks which drop into a magma as it ascends. 

 He has further developed this idea of abyssal assimilation, 47 until it re- 

 sembles the marginal assimilation hypothesis which is supported by many 

 French geologists. 48 If something of this kind has actually taken place 

 in the Cortlandt Series, it is probably a fairly common process, since it 

 has suggested itself in many parts of the world. 49 On the other hand, of 

 course, the location of these rocks may be merely an accident of intrusion. 

 Little can be said regarding the mutual relations of the four funda- 

 mental magmas, since they are not yet sufficiently clear. The volcanic 

 phase, the first in the normal cycle of igneous activity, is wanting. The 

 normal order of intrusions in the plutonic phase is that of decreasing 

 basicity, and this seems to have been followed in the Cortlandt. As al- 

 ready stated, the granites seem to have been intruded last, both from the 

 evidence of the acid dikes in the more basic rocks and from their almost 

 isolated position. The pyroxenites and norites, from their intimate rela- 

 tion in several places, were probably formed at about the same time, 

 although not simultaneously, for their association is only local. The 

 pyroxenites probably preceded the slightly more acid norites but had not 

 haidened when the latter appeared. At the same time, however, the evi- 

 dence of the variation diagram, as indicating a much more complex rela- 

 tionship, must not be lost sight of. The diorites were probably still later. 

 Here again, however, we are confronted by the suggestion of hybridism in 

 the occurrence of gradation from hornblende norite into diorite, for ex- 

 ample, and a further discussion of these problems would be fruitless in 

 view of the comparative scantiness of our data. 



""The Mechanics of Igneous Intrusion," Amer. Jour. Sci., (4), XV, 269, and XVI, 

 107. 1903. Also "The Geology of Ascutney Mountain, Vt.," Bull. 209, U. S. G. S. 1903. 

 11 Amer. Jour. Sci., (4), XXII, p. 195. 1906. 



48 See for example "Contribution a l'etude du granite de Flamanville," Bull, carte geol. 

 France. 1893. 



49 See M. Weber, k. Bayerischen Akad. der Wissenschaften, Dec, 1910. 



