IGNEOUS ROCK-SERIES AND MIXED ROCKS 



395 



Fig. 2. 



in the mixture be a to b, then Om will represent the silica- 

 percentage of the mixture, m being the point which divides 

 MM' in the ratio b to a. (The empty portion of the diagram is 

 omitted to save space.) If PpP' be the curve of variation of 

 some one of the bases, then mp' in the figure represents the per- 

 centage of that base in the mixed 

 rock. The percentage in the 

 corresponding member of the 

 rock-series is represented by mp, 

 and the mixed rock is therefore 

 deficient in this base as com- 

 pared with the latter, the defect 

 being represented by pp' . Simi- 

 larly a different base, having 

 QqQ' for its c curve, will be in 



excess in the mixed rock as compared with the corresponding 

 member of the rock series, the excess being represented by qq' . 

 It is evident that there will be a defect or an excess according as 

 the curve is convex or concave between the points corresponding 

 with the two component rocks. The defect or excess will be 

 greater, ceteris paribus, the farther apart the two component rocks 

 are in the series. It is easy to see that, given a series such that 

 its diagram has markedly curved lines, the result of the admix- 

 ture of two members may be something not only foreign to the 

 series, but highly peculiar by comparison with igneous rocks in 

 general. 



This may be still more strikingly the case in the admixture of 

 two rocks which have no such close relation with one another. 

 In illustration we take two simple cases of accidental xenoliths. 

 First suppose a rock-magma to become enriched in silica by dis- 

 solving quartz, of extraneous origin, a parts of the magma taking 

 up b parts of quartz. Dividing MX at m in the ratio b to a, 

 we have Om to represent the silica-percentage of the resulting 

 mixed rock (Fig 3). If MP, MQ, etc., represent the percent- 

 ages of the various bases in the original magma, then mp' , mq' , 

 etc., will represent them in the mixed rock, these ordinates being 



