404 A. C. LAXE POKPHYRITIC APPEAR AXCE OF ROCKS 



The distances of a series of sections from the margin of the dike or 

 sheet. One should be at the margin and others within a very few 

 inches. Another group should be from a quarter to half way from mar- 

 gin to center, one or two nearer the center, and one as near the center 

 as possible. 



The observations to be made in the laboratory include the size of the 

 different constituents in the different specimens, the average linear di- 

 mension of fair grains: but tiiis should not be divorced from a study of 

 signs of corrosion, transportations while being formed, floating in the 

 magma, and definite orientation relative to the margin. 



The results which may be looked for are : 



An idea of the size of the sheet and the contact zone, if these could 

 not be observed in the field ; 



An idea of the stage of development of the magma at the time of in- 

 trusion ; 



And. from the known limits of temperature or conditions of formation 

 of the constituent minerals, an idea of the relative temperatures of the 

 intruded and intruding rocks. 



