550 



OGILVIE 



Slides of the three smaller dikes into which the large one 

 appears to ramify in the middle of its extent show a similar 

 diabasic structure, but a much finer grain and a porphyritic 

 tendency. They show a felty appearance with fine feldspar 

 needles in a mass of magnetite, with very tiny augite anhedra, and 

 porphyritic plagioclase. The phenocrysts are of essentially the 

 same size as the average crystals of the center of the large 

 dike; the fineness of the ground-mass is the essential difference 

 between the two. 



In Washington's tables there are ten rocks within this subrang, 

 none of which comes from eastern America. Several are dikes 

 from California, but the one which is the closest analogue in 

 analysis and in norm is a porphyritic lava from the St. Augustine 

 volcano, Alaska. 1 



Table XII. 



Analysis of Dike from Eastern Shore of Linekin Bay. 



Composition. 



Molecular 

 Proportions. 



Norm. 



Si0 2 



53-oi% 



.883 



Qu 



4.20 



A1 2 3 



i5 



54 





152 



Or 



3-34 



Fe 2 3 



1 



85 





on 



Ab 



20.44 



FeO 



6 



09 





096 



An 



29-75 



MgO 



7 



70 





192 







CaO 



10 



60 





189 



Diop 



18.40 



Na 2 



2 



37 





039 



Hyp 



18.92 



K 2 





62 





006 



Ilm 



3-°4 



H 2 + 





73 





Mag 



2-55 



H 2 0- 





47 









co 2 



none 









Ti0 2 



1.70 



.020 







P 2 5 



trace 









s 



trace 









Total 



100.68 







' 



Auvergnose. III. 5. 4. 3. Diabase. Occurrence. — This rock 

 is found in the form of a dike on the eastern side of Linekin' s 

 Bay. The dike is about ten feet wide near the shore; it may be 

 followed for some rods inland, until it disappears under vegeta- 

 tion. The most vigorous search did not bring it to light farther 

 east. Westward it re-appears on Cabbage Island, where it has 



> See Becker, Annual Rept. U. S. G. S., XVIII, Pt. Ill, p. 52. 



