THE REACTION PRINCIPLE IN PETROGENESIS 



179 



temperature falls the amount of crystalline matter increases and at 

 the same time changes in composition along the solidus curve. 

 Thus at 1370° the crystals have the composition AbiAua. In other 

 words, the first-formed crystals have suffered a change of composi- 

 tion; indeed, they are continually modified in composition by 

 reaction with the Hquid. Crystals and Hquid mutually influence 

 each other throughout 

 the course of crystal- 

 lization. 



All of this is now an 

 old story and is repeated 

 here merely to empha- 

 size the marked differ- 

 ence in crystalKzation 

 between a mixture in 

 such a system and one 

 in a eutectic system. 

 In this latter, a crystal 

 once separated is no 

 longer concerned in the 

 equilibrium, which 

 makes it in every respect 

 a special case whose 

 occurrence might be expected to be comparatively infrequent, 

 particularly in more compHcated solutions (magmas) . 



On account of the continual reaction relation between crystals 

 and liquid in a solid solution series such as the plagioclases, it is 

 proposed, for the purposes of the present paper, to call such solid 

 solutions a continuous reaction series. The term will apply to any 

 solid solution series, whether to a complete series such as the plagio- 

 clases or to each separate series in the case of incomplete solid 

 solution. 



The essential feature of a reaction series, the reaction relation 

 of crystals and hquid, is retained when the series becomes a part 

 of a more complex system. This is true even when the end members 

 of the series bear a eutectic relation to the newly added component, 

 as is well shown when diopside is added to plagioclase. The 



Fig. I. — Eqmlibrium diagram of the plagioclase 

 feldspars. 



