6-lG PROCEEDINGS OF THE NEW YORK MEETING 



The coarsest country rock is a hornblende-gneiss. Irat the individual grains 

 are rarely over 2 to 3 millimeters across. A tendency to porphyritic texture, 

 like that of the Republic granite, is rare and not pronounced.* 



In but few places does the hornblendic country rock fail entirely to be 

 cut by red dikes composed mainly of quartz and feldspar. There is but little 

 sign of one being in excess over the other, and they are probably nearly 

 eutectic. 



Generally the rock is riddled with them and they stand in relief, and may 

 show glacial striie when the rest of the rock does not. They are close welded 

 to the country rock, so that it is not very ditficult to get a piece showing both 

 sides of the contact. Indeed, frost will sometimes spall off a section clear 

 across a small dike including a little of the comiti-y rock on either side. Some- 

 times, as at the top of Huron mountain, they are so abundant that at first 

 sight the country rock appears like a lot of basic fragments or secretions in- 

 closed in a general mass of granite : yet it is very easy to see that through 

 all these apparent fragments there is a constant strike and dip of the schis- 

 tosity in which the red granitic matter does not partake. A more careful 

 examination, moreover, shows the nexus of dikes which make up the general 

 granitic effect. 



Now we may for convenience sake divide these dikes into (a) those whose 

 grain is finer than the country rock, less than 2 to 3 millimeters, often about 

 1 millimeter, which we will call aplites; (6) those whose grain is coarser 

 than the country rock, often 50 millimeters or so, which we ma5' call pegmatites. 

 Exceedingly coarse pegmatites I did not see. 



We find the following variation of grain in individual dikes : 



1. Dikes aplitie in general grain, with coarser border. 



2. Dikes aplitie. but with coarser pegmatite bands near, but not at, the side. 



3. Dikes granitic in grain, with coarser pegmatitic border. 



4. Dikes with a fine-grained margin and coarser center. This type seems 

 rare, and in many, but not all, cases may be explained by the splitting of a 

 fine-grained dike by a later coarse-grained one. 



5. In many cases the grain seems uniform — at least to casual inspection — 

 throughout. 



It is not uncommon to find a concentration of the darlcer minei'als, the 

 mica or hornblende, at the center of the dike. 



A very striking dike of pegmatite (figure 1) on mount Homer, 28 inches 

 wide, shows irregular prisms of quartz growing from the margin, while the 

 center is made up of patches of graphic granite. Figure 1 is, however, only 

 a memory sketch from notes at the time.f 



In a rough way the larger pegmatites are also coarser in grain ; but while 

 both the pegmatites and the aplites are close welded and have about the 



* I noticed a trace with feldspar grains 10 millimeters long on the south side of 

 Second Pine lake. A splendid porphyritic rock at the foot of mount Homer, and near 

 J. M. Longyear's house, on Ives lake, I take to be a Huronian dike. 



t I remember seeing a similar structure in a pegmatite dike on the north side of Sand 

 lake, Proudfoot township, Parry sound, Canada. The dike was .32 inches broad, and 

 quartz radiated from each side for about 4 inches, while the dike as a whole was made 

 up of biotite, muscovite, feldspar, and quartz in good-sized crystals. 



