PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 559 



Diabase.] 



also borders the grains, and sometimes extends through quite a portion of their mass. 

 The olivine is foreign and possibly the magnetite is the same; but if not, it has been the 

 first mineral to crystallize from the magma followed by the feldspar and lastly by 

 the augite, although the last two were nearly, if not quite, contemporaneous. 



"In one portion of the section the twinning of the plagioclase is seen to be 

 dependent upon the pressure exerted by the solidifying augite." 



The olivine, in part at least, is of later date than the feldspar. One section. 



Age. Cabotian. u. s. G. 



Remark. In respect to the comparative dates of the olivine and the other min- 

 erals, this rock resembles Nos. 258, 512, 703, 757, 1275 and 1842. The augite is 

 ophitic. N. H. w. 



No. 820. DIABASE. 



Prom the railroad cut, Taylor's Falls. 



Eef. Annual Report, x, pages 106, 116-118; Final Report, vol. i, pages 196-199. 



A tough, heavy, dark-gray rock, with medium grain, not much changed, 

 the weathered surface specked by the weathering out of ophitic pyroxene. 



Mi<: The rock is considerably altered, but the ophitic relation of the feldspar 

 with the pyroxene is beautifully exhibited. The feldspars are in two sizes or dates 

 of origination, the microliths being embraced in the later formed pyroxene. Yet this 

 distinction into two generations is not common. In general the feldspars are of one 

 size, and are considerably altered, giving rise to calcite, Muscovite and pennine. Some 

 calcites are large. 



The pyroxene grains are so large that, with their included microliths of feld- 

 spar, they individually, sometimes, cover the whole field of the microscope (object- 

 ive No. 3) and embrace from twenty-five to fifty of the small feldspars. In no rock 

 have I seen this structure so well displayed (see photograph figure 12, plate I). The 

 pyroxene is of the form augite when not altered to hornblende. 



The olivine grains are small, and doubtless of the first generation, frequently 

 surrounded by magnetite, and so decayed that they have no effect on polarized light, 

 being then greenish. 



Leucoxene and epidote, with more or less chlorite, serve to becloud the section, 

 especially when thick. 



Magnetite is quite common, but for the most part is later than the other minerals, 

 prevailing in the neighborhood of the olivine. 



Much of the augite, of a date earlier than the large ophitic plages, is surrounded 

 by a faintly yellow border that is apparently due to alteration. It has different 

 orientation from the augite and probably is related to hornblende. Three sections. 



Age. Cabotian. N. H. w. 



