I»2 



WILLIAM H. HOBBS 



Possible formation of a magma macula beneath an anticline. — 

 From examination of Fig. 17 and Table I (pp. 178 and 175) it will 

 be seen that a relatively competent member near the top of a series 

 of beds may remove from underlying members as much as one-half 

 of the load which rests upon the arch. Under such conditions the 

 underlying beds may be deformed by failure, even though the 

 competent member is not (Fig. 25), and if at a depth where the 

 isogeotherms are sufficiently high to melt the inferior members when 

 thus partially relieved of load, a macula of magma may develop 

 from the fused sediments. Such fusion is the more Hkely to occur 



Fig. 24. — View of the front of the ice in northeast Greenland where overturned 

 anticlines with thinned upper limbs are to be seen fafter Koch and Wegener). 



the lower the fusion interval of these inferior formations. The 

 writer has elsewhere shown that the composition of igneous rocks 

 in general is such that magma must inevitably develop, if at 

 all, from the pelitic sediments, like shale and slate; the great 

 abundance of which, no less than their structural weakness and 

 their normal position in sedimentary series, is in favor of the view.^ 

 Reduction of volume of anticline an efficient cause of elevation of 

 magma. — Before the formation of a macula of magma through fusion 



^ " Some Considerations Concerning the Place and the Origin of Magma Maculae," 

 Gerland's Beitrdge zur Geophysik, XII (1913), 329-61, Figs. 1-8; see also ''Variations 

 in Composition of Pelitic Sediments in Relation to Magmatic Differentiation," 

 Comptes rendus 12^"^ Congres Geologique International, Canada, 1913. 



