12 Weed and Pirsson — Highwood Mts. Laccoliths. 



Our interpretation of the facts previously described then, 

 is, that if one were to take the cross section of fig. 10 and 

 revolve it upon a perpendicular drawn through the middle 

 point, the resulting figure of revolution which would be gen- 

 erated would exhibit our conception of the laccolith and the 

 structural relation of its interior parts. It is true this would 

 cause the laccolith to become a true circle in ground plan, and 

 the successive shells and syenite kernels to be also circular in 

 ground plan or projection on a horizontal map surface, and to 

 have a common center — and we do not know that this is 

 exactly the case ; the laccolith may be more or less ellipsoidal 

 or irregular in outline and so also the interior shells and the 

 kernels ; they may not have exactly common centers nor be 

 everywhere of the same thickness, and this is very probably 

 the case ; nevertheless, these are mere details and we believe of 

 little importance in comparison with the idea that the figure 

 would express the generally circular, concentrically zonal 

 arrangement of the parts. 



While the discussion of the facts presented and the conclu- 

 sions drawn from them in their special relation to recent 

 theories in the field of petrologic research would carry us 

 too far for the purposes of this paper, it will be well to point 

 out here some of the important bearings they have in this 

 respect. 



In the first place, it appears clear to us that the structural 

 relations of the different parts of the laccolith to each other 

 are due to causes which have operated in the laccolith itself. 

 We conceive that the body of the magma forming the lacco- 

 lith must have been injected as a whole, in a homogeneous 

 condition, and that rearrangement and formation of the various 

 parts followed within the mass itself. The occurrence of ball- 

 like masses in the upper crust of the laccolith seems to show 

 that the filling took place with considerable rapidit}?. We 

 believe that after injection of the homogeneous magma, the 

 first stages of cooling and crystallization against the outer 

 envelope of sedimentary rocks was relatively more rapid than 

 that affecting the inner portion, and resulted in producing the 

 outer porphyritic shell, the zone of endomorphic contact meta- 

 morphism. In the meantime, and as time went on, there was 

 a gradual withdrawal and concentration of lighter feldspathic 

 material by some process of differentiation, to the inner upper 

 portion of the mass, with a relative enrichment of an outer 

 zone in lime, iron, and magnesia. This is most marked in the 

 case of the white, inner syenite, and naturally less so in the 

 transition rock. 



After the process had finished or perhaps while it was still 

 going on, crystallization and solidification were advancing from 



