392 /. MORGAN CLEMENTS 



kinds of rocks, it was seen that the hornblende-gabbro was 

 unquestionably the one which first reached its present posi- 

 tion. Whether this is to be regarded as itself representing the 

 composition of the parent magma, or only as a differentiation 

 product of a still deeper seated igneous mass, cannot of course 

 be determined. Be that as it may, the fact, which has been 

 proven, remains, that given the period of eruption of this horn- 

 blende-gabbro as a starting point, and possibly this magma as 

 the original one, the forces of differentiation have been active in 

 two directions, towards increasing acidity and increasing basicity, 

 in agreement with the law of succession of igneous rocks as pro- 

 pounded by Iddings. 1 



J. Morgan Clements. 



1 The Origin of Igneous Rocks, by J. P. Iddings, Bull. Phil. Soc. Wash., Vol. XII,. 

 1892, p. 145. , 



