86 



University of California. 



[Vol. 2.- 



eruptions of each period were generally the most basic, and that 

 the later intrusions were the most acid." 



Teall * says : " Given one reservoir and continuous cooling there 

 would be one sequence, the basic rocks should precede the acid. 

 This has been the case in many regions, but the rule is by no 

 means without exceptions. The exceptions may be due to the ex- 

 istence of two or more reservoirs or to the accession of heat or of 

 fresh material during the process of consolidation." 



In a paper by Geikie and Teall f there are some interesting ob- 

 servations on the differentiation of magmas: "We often find what 

 is a connected series of intrusions exhibiting a wide range in chem- 

 ical and mineralogical composition, and certain definite though 

 probably not very great differences in age. Such a plutonic area 

 taken as a whole, though forming a petrological complex, is a geo- 

 logical unit. The complexity in such cases can not wholly be 

 accounted for by differentiation in situ. The more abrupt changes 

 require the hypothesis of successive intrusions, which probably rep- 

 resent differentiation elsewhere of the same general character as that 

 which has taken place to a smaller extent in situ. We recognize, 

 therefore, two kinds of petrological complexity in plutonic areas, 

 (i) differentiation in situ, (2) successive intrusion." 



In the study of the Yogo Peak igneous rocks, PirssonJ found 

 a progressive differentiation from one end to the other of a large 

 intrusive mass, to which he applied the term " facies suit," after 

 Brogger, who used it to designate the differentiation of a mass in its 

 final resting place. Mr. Pirsson also found in the rocks of Square 

 Butte, in Montana, a progressive increase in the ferro-magnesian 

 silicates toward the outer border. 



Brogger § reaches the conclusion that there is a fairly common 

 sequence. " The series basic, less basic, acid appears so often 

 among the deep-seated rocks that we must look upon this series as 

 a normal one; the sudden return to the basic is known in many 

 occurrences, but appears to fail often. . . . The deep-seated 



* Nat. Sci., Vol. I, No. 4. 



f Quart. Jour, of the Geol. Soc 1894. 



% Am. Jour, of Sci., Vol L, p. 467. 



'i Die Eruptionsfolge der Triadischen Eruptivegesteine in Sudtyrol. 



