CHEMICAL RELATIONS OF A PETROGRAPHIC PROVINCE 693 



which it was obtained. In a similar manner the base on which 

 the model rests is made to project a short distance at one end, so 

 that another set of labels showing the chemical constituents repre- 

 sented in the analyses may be placed upon it. In this way, look- 

 ing down the length of the model the character of the variation in 

 the content of any chemical constituent in the series of rocks 

 composing this petrographical province can be seen at a glance. 



The predominance of any constituent, or group of constituents, 

 in a certain part of the series is shown by a hill rising from the sur- 

 face of the model, the shape of the hill varying according to minor 

 variations in chemical composition of the rocks of this portion of 

 the series. Depressions, on the other hand, indicate low per- 

 centages of a constituent, or group of constituents, in certain 

 portions of the series. 



The model shown in Fig. 2, representing the chemical relations 

 of the rocks constituting the Monteregian Hills, combines the results 

 of 36 analyses, the rocks ranging in composition from the acid 

 nordmarkite of Mt. Shefford, containing 65.43 per cent of silica, 

 at one end of the series, to the basic alnoite of Point St. Charles, 

 near Montreal, holding 29 . 24 per cent of silica, at the other end. 

 The intervening portion of the model shows the chemical composi- 

 tion and the mutual relations of the magmas of intervening acidity 

 represented by the pulaskites, nephehne syenites, essexites, rouvel- 

 lites, tinguaites, camptonites, montrealites, tawites, rougemontites, 

 yamaskites, monchiquites, etc., of this petrographical province. 



The model is 36 inches long, 12 inches wide, and 8| inches high 

 at the highest part. It appears to present in a clear and rather 

 striking manner the chemical relations of the alkaline magmas 

 of this peculiar petrographical province, and similar models might 

 be readily constructed which would set forth the characteristic 

 relations of other provinces. 



