PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 907 



Qabbro.J 



as they sometimes embrace the much altered feldspars in an ophitic manner. This 

 rock has less quartz than the last, but what it has is secondary to the other minerals 

 as in that. It contains also much calcite. Biotite and chlorite are interleaved. 

 Two sections. 



Age. Cabotian. 



Remark. This rock is in a secondary state. The most of its minerals are such 

 as are known to be the products of alteration. The introduction of the quartz, 

 biotite and calcite seems to have been simultaneous with the alteration of the 

 plagioclase. 



There may have been a primordial basic magma of the ordinary diabase type, 

 which rose near the surface of the earth at the time of the Cabotian disturbance, 

 but coming into contact with the clastic crust, which was necessarily more acid than 

 itself, was modified by it during the process of cooling. This modification, so far as 

 evinced by the composition of rocks Nos. 854G and 854aG, and the altered condition 

 of the primordial minerals, consisted in the total loss of the ferromagnesian minerals 

 (augite and olivine) and of any magnetite or apatite that may have been in the 

 primordial magma, and the introduction of acid elements, viz., quartz, orthoclastic 

 feldspar and muscovite. Calcite may have been derived from the simultaneous 

 alteration of the original labradorite. These changes were a " secondary " process 

 so far as concerns the original basic rock, but the minerals that resulted are as old 

 as the rock itself, considered as a solid body, and in that sense they are original to 

 the rock. These alterations therefore are believed to be not due to weathering nor 

 to dynamic action, nor to contact metamorphism of any igneous rock of later date 

 than that of the rock itself. These "secondary" minerals are more stable, as a 

 group, than the minerals which they have replaced. They are quartz, biotite, calcite, 

 muscovite; and such minerals as a group endure weathering and all dynamic action, 

 as is well known, more successfully than olivine and augite. Throughout the body 

 of the diabase and gabbro mass in Minnesota, wherever olivine and augite are found 

 at all and have not been subjected to abnormal contacts or dynamic action, they are 

 apparently as fresh and clear as when they were first formed. They have not suffered 

 any deep-seated, metasomatic alteration. That fact not only shows that they have 

 endured intact since Taconic time, but also that it is only in abnormal conditions 

 resulting from igneous contact and its heated solutions under dynamic stress that 

 they are lost to the rock. N. H. w. 



No. 857G. GABBRO. (Olivinitic.) 



Northeast shore of bay at north side of Bashitanaqueb lake, near the north line of sec. 2, T. 64-5 W. 

 Ref. Annual Report, xxi, pages 66, 150, 151. 



Meg. A fine-grained, dark-gray, granular rock composed of feldspar and dark 

 minerals. 



