462 



JOSEPH BARREL'L 



done is to interpret the deflections as produced by spherical masses. 

 What then are the geological suggestions as to whether vertical 

 prolate or oblate forms may be expected to characterize the larger 

 outstanding masses ? In the one case the error of interpretation 

 will be to make the masses appear too small and shallow; in the 

 other case, to make them appear too great and deep. 



TABLE XXVI 

 Errors Due to Interpretation as Spheres of Unit Masses at Depth D 



Form 



Data 



Fit Max. 



EM. 



Assumption 



Resulting 

 Interpretation 



Depth to 

 Center 



Mass 



Case C (True) 



Interpretation as a sphere 



Case D (True) 



Interpretation as a sphere 



4-5 

 4-5 

 3 05 

 3 05 



74 km. 

 74 km. 

 27 km. 

 27 km. 



41 



55 

 67. 



55 



i.oD 

 1.6D 

 i.oD 

 0.6D 



i.oM 

 1.8M 

 i.oM 

 0.6M 



Stocks, and especially volcanic pipes, approach in form to vertical 

 cyHnders, but these are merely connecting structures. On the 

 other hand, mountain ranges and geosynclines, although linearly 

 extended, are of breadth which is great in comparison with the 

 depth of excess or defect of density. Laccoliths and regional 

 extrusions are also broad in comparison with depth. The relations 

 as regards the great intrusive masses are not so clear, but erosion 

 exposes batholiths over progressively greater areas; and whole 

 provinces which exhibit regional metamorphism give suggestions 

 that they are underlain by widespread igneous bodies. The 

 hydrostatics of the magmas and their diiferentiation into masses 

 of unhke density would also give tendencies to layers and horizontal 

 extensions of the larger masses of abnormal density. These would 

 depart, then, from the form of spherical masses in the direction of 

 oblate spheroids with their equators in a horizontal plane. Nar- 

 rower belts of disturbance like that which passes through Washing- 

 ton, D.C., may, on the other hand, tend to have the form of vertical 

 plates. Therefore in none but the smaller and connecting struc- 

 tures are there geological suggestions of vertical prolate form. 



