300 A. Wandke— Study of Cape Neddick Gabbro. 



The banding in igneous rocks has received considerable 

 attention from petrographers and many examples have 

 been described in detail. Various theories have been 

 advanced to explain these banded rocks and it seems quite 

 certain that different processes have been operative in 

 different localities to produce a somewhat similar result. 

 The entire subject has been well reviewed by Dr. F. F. 

 Grout 2 who gives the following ways in which these 

 banded rocks may have been produced: 



1. Partial assimiliation of inclusions forming schlieren. 



2. Lit par lit, or fluidal gneiss. 



3. Deformation during solidification. 



5. Streaked differentiation with reference to rhythmic cooling 



or intrusive action. 



6. Successive intrusions. 



(a) Cooling separately and successively. 



(b) Cooling later, all together. 



7. Heterogeneous intrusion. 



8. Convection during crystallization differentiation. 



At Cape Neddick where the bands are rarely over three 

 inches wide, stand vertically, have a granitic texture, 

 show but slight variations and can be followed for 

 hundreds of feet about the contact, most of the above 

 listed explanations such as 1, 2, 3, 6, and 7, can obviously 

 be dismissed as non applicable. The facts needing expla- 

 nation are the localization of the banded rock near and 

 parallel to the contact; a vertical banding; alternation 

 between femic and salic rock types; the alteration of 

 pyroxene to hornblende and biotite with partial absorp- 

 tion of magnetite; the passage of banded gabbro into 

 unhanded gabbro toward the center. 



The narrow width of the bands and the passage from 

 banded rock into unbanded rock would seem to rule out 

 the possibility of successive intrusions. There remain 

 thus as the favored hypotheses either convection during 

 crystallization differentiation or rhythmic 3 cooling. 

 Were it a question of explaining a banding that showed 

 gradations from basic to more acidic rock types or vice 

 versa then convection might have played an important 

 part. A study shows that the bands differ only in the 



2 Journal of Geology, vol. 26, p. 439, 1919. 



3 N. V. Ussing : Geology of the Country Around Julianehaab, Greenland, 

 p. 361, 1911. 



