MINERALOGY AND PETROLOGY. 981 



Structural field relations.] 



above mentioned may have one and some another of these methods of derivation, 

 independently of an intermediate acid state of the greenstones from which they were 

 derived. 



1. Complete fusion of contacting acid rocks and the perfect mutual transfusion 

 and molten mixture of the two kinds into a homogeneous magma. 



2. The penetration of the acid element into the basic along contacting zones 

 through the agency of mineralizers, during the period of cooling. 



3. Submarine extrusion, attended by oceanic precipitation of silica. 



Some of the massive quartz-gabbro and augite syenytes appear to have had the 

 first mentioned method of origin. 



To the second method may be assigned those that show the micro-pegmatitic 

 structure, and the uralitic state of augite, with chlorite and a reddened state of much 

 of the feldspar. 



The third class would include much of the " red rocks." When such basic 

 extrusions were in great amount and were able to crystallize before modification, the 

 resultant rock may have been such as seen in the Misquah hills. When the extru- 

 sions were in small amount, and were solidified before crystallizing, the glassy mass 

 was wholly silicified. Thus resulted jaspilyte and probably much of the apobsidians 

 of the Keweenawan. 



1. Structural field relations. In repeated instances the recognized gabbro mass 

 has been seen to grade into noryte, which (in its normal and usual type) is one of 

 the forms of niuscovadyte, the latter term being more flexible and general. This fact 

 is attested by many observers, whether in Minnesota or in Canada, and need not be 

 dwelt on to any length. Yet this noryte has no more stable composition and char- 

 acters than gabbro, when considered as a rock mass, and in some of its variations it 

 loses its right to the name noryte, and the rock has been given other designations. 

 If the structural relations of noryte to the .coarser gabbro be examined it will be 

 found that the transition is both gradual and sudden. The gradual transition is that 

 which has been most frequently observed (Nos. 983, 984), but there are many places 

 along the northern border of the normal gabbro mass where the coarser gabbro rock 

 exhibits a non-conformable contact on noryte masses, i. e., on what have been denom- 

 inated niuscovadyte masses. There is a figure in volume iv (page 303) which illus- 

 trates this fact, although, in that instance, the included masses are not typical noryte, 

 but approach a granulitic gabbro. At the time the sketch was made* no specimens 

 were collected at this point, but later specimens (No. 847G) were obtained from the 

 angular included mass by Dr. Grant, who makes the following remark :f 



*By DR. A. WINCHELL. See Fifteenth An nun I /vc/""'',P- 1"2. 

 t Twenty-Fourth Annual Report, p. 127. 



