Metamorphic and Plutonic Rocks at Umeo. 119 



nine to ten feet in thickness, and striking N. 10" E. There 

 IS also here a strong dyke of basic rock on the same strike, 

 which I did not further examine. 



The crystalline granular character of these rocks raises a 

 doubt whether they may not be, in fact, members not of the 

 schist group in a metamorphosed condition, but of the 

 granites. Yet their resemblance in some respects to portions 

 of the most altered of the schists which I have described, 

 and the absence of any defined contact, cause me to hesitate 

 as to the class to which I should assign them. I shall again 

 refer to this, after describing the remainder of the section, 

 and I now proceed to give some data as to the mineral 

 composition of these rocks at (771). 



The dyke of " graphic granite " is a good example of one 

 of the extreme forms in which the granites of this neigh- 

 bourhood not infrequently occur. It seemed to me to be 

 worth further examination. It is light-coloured, or of a light 

 yellowish tint. It has a platy structure in places, due to a 

 tendency to split along the cleavages of the felspars, which 

 are similarly oriented over considerable spaces, for instance, 

 over several inches square. The larger part of the rock 

 seems to be felspar, and the lesser part quartz, in grains 

 and in veinlets, producing in the planes of separation those 

 figures which have given a name to this kind of rock. 

 Where there are fissures traversing the rock, secondary 

 muscovite has been produced. 



Examined in a thin slice, I found this rock to be com- 

 posed of microcline and quartz, with a very little secondary 

 muscovite mica. The microcline is twinned in the well-known 

 manner, and contains portions which are not twinned, and 

 which re-act with polarized light, as does the monoclinic 

 potassa felspar. Albite is in considerable amount, and occurs 

 in veinlets, in small twinned crystals in the microcline, and 

 more rarely outside of it. The quartz has no crystalline 

 form, but is in irregularly-shaped masses, such as are well 

 known in graphic granite. The rock is traversed by fissures 

 which have been filled partly by the comminuted felspar, and 

 partly by secondary muscovite resulting therefrom. Round 

 some of the quartz grains there are radiating cracks and 

 disturbances of the microcline twinning, indicating strains. 



On these data, this rock may be described as a gTaphic 

 granite, and from its occurrence as a dyke at this place, may 

 be considered allied to, but, in all probability, younger than 

 the aplites. 



