468 SCIENCE PROGRESS. 



The same authors have studied certain allied rocks 

 occurring- at Yogo Peak in the Little Belt Mountains, also 

 in Central Montana (12). These rocks form parts of a 

 mass two miles long and one mile wide occupying a great 

 fracture in the Palaeozoic strata. At the eastern point of 

 the peak occurs a syenite with 61^ per cent, of silica and 

 moderately high percentages of magnesia and lime. It is 

 an augite-syenite, though with subordinate hornblende. 

 Westward it gradually changes character in the sense of 

 becoming more basic, until at the summit it contains as 

 much augite as orthoclase. This type, which has about 54^- 

 per cent, of silica, has been named yogoite. At the 

 western point of the peak the augite predominates over 

 the orthoclase, while biotite and iron-ore also become 

 prominent, and pseudomorphs after olivine are sometimes 

 found. The rock here is a shonkinite similar to that of 

 Square Butte. The silica percentage has fallen to 49, while 

 the amounts of iron-oxides, magnesia and lime have increased 

 considerably as compared with those in the syenite. A 

 still more basic type occurs in irregular masses at the 

 contact. In Yogo Peak then we have an intrusive stock of 

 oval form, which shows a progressive differentiation from 

 east to west along its major axis. It may be remarked that, 

 as the several associated rock-types are composed in general 

 of the same minerals, the augite in particular running 

 through all the varieties, the differentiation seems to have 

 been of a kind which has elsewhere been considered to 

 have been effected concurrently with, and as a consequence 

 of, crystallisation. Another interesting point is the occurrence 

 of shonkinite in association with two distinct types of syenite, 

 the sodalite-syenite of Square Butte and the augite-syenite 

 of Yogo Peak ; illustrating the fact that a given rock- 

 type may originate by differentiation in more than one way. 

 A series of specimens, chiefly from intrusive sheets, in 

 the southern part of the same state have been described by 

 Merrill (13), and some of them show evident affinities with 

 the foregoing rocks. One type, described in several 

 examples under the name augite-porphyrite, compares 

 rather closely in chemical composition with the yogoite of 



