Lawson.] 



Orbicular Oabbro. 



391 



Encasing this core is a succession of shells which in thin 

 section appear to be much more sharply marked off' from one 

 another than would be suspected from a macroscopic inspection. 

 This distinctness of the shells is due to the fact that they are in 

 reality an alternation of thick shells composed of feldspar and 

 olivine in radial disposition and very thin shells composed wholly, 

 or almost wholly, of feldspar. The feldspar in all cases appears 

 to be bytownite or anorthite as determined by the usual optical 

 method. These thin shells of feldspar may be regarded as repre- 

 senting interruptions in the growth of the olivine. The occurrence 

 of the latter mineral is one of the most peculiar features of the 

 orbuies. It occurs in slender rods of varying thickness disposed 

 radially. In the thick shells these olivine rods traverse the entire 

 width of the shell as seen in section and stop abruptly upon the 

 thinner shells composed wholly, or almost wholly, of feldspar. 

 Where these thin shells or feldspathic partings between the 

 thicker shells are well defined, an occasional rod of olivine may 

 cross them, but this scarcely detracts from the sharpness of the 

 shell limits. In cases where the thin shell is only feebly devel- 

 oped, the olivine rods cross it rather commonly, but they then 

 often show an abrupt diminution of their thickness where they 

 cross the shell, and the trace of the shell is still well marked in 

 thin section and contributes much to the concentric appearance. 

 There is no even line of any kind between the feldspar of the 

 thick shells and that of the thinner shells or partings. The 

 crystallization of the feldspar appears to have been a continuous 

 process throughout the growth of the spheroids ; and the concen- 

 tric structure is due chiefly to the distribution of the olivine rods, 

 and particularly to the interruption of the growth of the olivine 

 rods at sharply defined spherical surfaces. The relative abun- 

 dance of the olivine in the different shells also, of course, 

 contributes to the accentuation of the concentric structure. 

 Thus in the outer shells the olivine is, in several cases observed, 

 more abundant than in those nearer the center of the orbuies. 



If the disposition of the olivine is the chief factor in the 

 concentric structure, the same statement is even more true with 

 reference to the radial structure. The feldspar has no special 

 radial disposition beyond that which is imposed upon it by the 



