BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 14, pp. 369-384, PLS. 54-58 SEPTEMBER 30, 1903 



STUDIES OF THE GRAIN OF IGNEOUS INTRUSIVES 



BY A. C. LANE 



{Read before the Society December 31, 1902) 

 CONTENTS 



Page 



Introduction . . 369 



Augite of the "greenstone " ophite 369 



Palisade trap 371 



Diabase dike at Light House point, Marquette 374 



Medford dike 376 



Suggestions as to comparative initial temperatures , 377 



Other minerals of the Marquette dike 377 



Franklin Furnace minette 378 



Explanation of plates 380 



Introduction 



It has seemed to me that it would be well in connection with the 

 following paper, which should be read first, on porphyritic appearance 

 to show how the principles and results there stated are used in particular 

 cases. I propose, therefore, in this paper, in the first place, to study the 

 grain of the augite in a group of chemically similar diabases. This is 

 the mineral which seems to lend itself best to mathematical treatment. 

 The comparative results are interesting and suggestive. I will, then, 

 take one of these rocks in Marquette and study the grain of all its con- 

 stituents to show how we must use our formulae, with due regard to 

 petrographic observations and scientific common sense. 



But first will be illustrated the methods of computation in the case of 

 one rock, which I take to be efi'usive, the (greenstone) ophite of the Isle 

 Royal report,* which is in chemical and mineralogical composition 

 similar to other rocks. 



Augite of the " Greenstone " Ophite 



We will take the observations as given on page 245 of the Isle Royal 

 report on thin-sections 15252 to 15258 and reduce the distances to milli- 



*Geol. Survey of Michigan, vol. vi, part i. 

 LII— BuLi,. Geol. Soc. Am.. Vol. 14. 1902 (369) 



