958 THE GEOLOGY OP MINNESOTA. 



[Hornblende fringes. "Original" hornblende. 



hornblende is cut by its cleavages, and it is only in rare instances, such as seen in 

 No. 1345, that the individual globules retain any distinguishing form or coloration. 

 Of all the rocks this feature is most common in muscovadyte. 



Hornblende fringes. That hornblende in all its varieties is invariably the result 

 of some secondary forces which have changed other minerals, usually augite, into 

 hornblende, is abundantly shown in the details of the foregoing descriptions. The 

 "porphyritic" hornblendes are simply developed fragments or crystals of augite. 

 This is true not only in some of the green schists, as those seen about the western 

 confines of Long lake (Nos. 2104, 2125), about Fall lake (No. 139W) and in the region 

 of Kekequabic lake (Nos. 1047, 1049), each of which contain other evidences of their 

 clastic origin, but even in the more massive bosses of "hornblende porphyry," and 

 in the narrow (camptonyte?) dikes, seen at Ely and at Stuntz island (Nqs. 872, 877, 

 1786), which are intrusive in the Keewatin. The most clear instance of the conver- 

 sion of augite to hornblende was seen in a Keweenawan diabase (No. 1847). Here 

 the hornblendic product forms a "fringe" surrounding the augite at its extremities 

 and having separate orientation. This enlargement in other cases is converted to 

 an encroachment, and gradually the whole augite grain is converted to hornblende 

 (No. 872), there being left occasionally only a small remnant, or several globularrem- 

 nants near the centre (No. 1060), to show the original augitic nature of the grain. 

 This graduated series of changes shows that the " fringe " is the first to form, and that 

 the latest hornblendic molecule was that which replaced the last augitic molecule. 

 That is, it is evident that the hornblendic fringes are not "frayed ends" of horn- 

 blende due to dynamic action, as supposed by Williams, nor yet "enlargements" of 

 original hornblende crystals, as suggested by Van Hise, but are the oldest parts of 

 the hornblende grains to which they belong. The outline of the original augite is 

 sometimes preserved by a patch of irregular shape occupying the central portion, 

 which has greater absorption than that which surrounds it, and also between crossed 

 nicols gives a different color of double refraction (Nos. 1047, 139W, 15H). In many, 

 and probably in most, cases, the augitic characters are wholly obliterated and horn- 

 blende appears to be an " original " mineral, due to the profound transformation 

 which the entire rock mass has experienced, as in dioryte. 



"Original" hornblende. Some hornblendes, in massive rocks, especially those 

 about Epsilon lake, have the aspect of being " original," i. e., that they formed in a 

 molten magma on consolidation. These are Nos. 751, 792G-797G. Some of these 

 have been carefully re-examined, with the following results: 



No. 751. So far as can be seen the hornblendes are original. 



No. 792G. Has distinct augite remnants within the hornblende. 



No. 793G. Shows some central nuclei or remnants, but not of augite. 



No. 793aG. Original, so far as can be seen, but with two or three central nuclei, though not of augite. 



No. 794G. Original, so far as can be seen; indistinct nuclei, as in No. 793aG. 



