bowen's hypothesis 407 



the Adirondack anortliosite, it is important for the reader to have this 

 hypothesis clearly in mind. In order to fairly and succinctly state this 

 hypothesis, I take the liberty of quoting a considerable portion of the 

 summary of Bo wen's first paper/ ^ as follows : 



"Aiiortliosites are made up almost exclusively of the single mineral plagio- 

 clase, and in virtue of this fact they present a very special problem in petro- 

 genesis. The conception of the mutual solution of minerals in the magma and 

 the lowering of melting temperature consequent thereon is no longer applica- 

 ble. Yet anorthosites give no evidence of being abnormal in the matter of 

 temperature to which they have been raised ; in other words, they give no evi- 

 dence of having been raised to the temperature requisite to melt plagioclase. 

 A possible alternative is that they may never have been molten as such, and 

 are formed simply by the collection of crystals from a complex melt, probably 

 gabbroic magma. ... 



"A consideration of the method whereby the accumulation of plagioclase 

 crystals might take place leads to the conclusion that the most promising is 

 the separation by gravity of the femic constituents from gabbroid magma, 

 while the plagioclase crystals, which are basic bytownite, remain practically 

 suspended. Then, at a later stage, when the liquid has become distinctly 

 lighter, having attained diorite-syenite composition, the plagioclase crystals, 

 which are now labradorite, accumulate by sinking and give masses of anortho- 

 site, at the same time leaving the liquid out of which they settle of a syenitic 

 or granitic composition. 



"Some of the consequences of this manner of origin of anorthosite are as 

 follows : Typical anorthosite, very poor in bisilicates, should not occur as small 

 dikes, for a mass of accumulated crystals should have little invading power. 

 . . . Typical anorthosite should for like reasons not occur as an effusive 

 rock. . . . Anorthosite should be intimately associated with gabbro, but 

 perhaps as intimately with syenite or granite. Anorthosites should commonly 

 be labradorite rocks rather than bytownite or anorthite rocks. 



"For the Adirondack area especially, evidence is adduced favoring the possi- 

 bility that there anorthosite and syenite may still occupy the relative positions 

 in which they were generated by the process outlined, the Adirondack complex 

 being interpreted as a sheetlike (laccolithic) mass with syenite above and 

 anorthosite below." 



In his second paper^^ Bowen modifies this hypothesis and interprets the 

 whole Adirondack igneous complex as a tremendous laccolith, with essen- 

 tially a stratiform arrangement of pyroxenite and gabbro below; next 

 above, anorthosite, and an upper chilled gabbroid border, the considera- 

 tion of a chilled border having been a result of Cushing's rejoinder to 

 Bowen's first paper. 



In the development of his hypothesis, Bowen also stresses the follow- 

 ing: (1) Common occurrence of rocks intermediate between syenite and 



11 N. L. Bowen: .Tour. Geol., vol. 25, 1917, pp. 242-24.3. 

 12 N. L. Bowen : .Tour. Geol., vol. 2.5, 1017, pp. 512-514. 



