414 THE VERMILION IRON-BEARING DISTRICT. 



the gabbi'O, and of a relatively small mass and quick cooling and crystal- 

 lization, as in the case of the sills. This difference in the rapidity of cool- 

 ing is shown also by the porphyritic structure of the rocks of the sills and 

 the fine-grained rock occurring upon the edges of the sills. The relative 

 rapidity of the cooling is further indicated by the feldspar phenocrysts 

 which occur in the sills. These very commonly reach a length of 2 inches, 

 with a maximum length of 4 inches. Had the magma cooled under the 

 conditions under which the phenocrysts were formed, obviously the rest of 

 the constituents would have also attained much larger size than they did, 

 and the resulting rock would have been coarse grained and doubtless as 

 granular as any of the gabbro. 



But let us not neglect the evidence presented in the above-quoted state- 

 ment itself — that offered in the statement that there are sills at Akeley 

 Lake which are apparently of gabbro and not, like the other sills, different 

 from the gabbro. Let us attempt to conceive of the conditions under wliich 

 these sills were formed. They were injected into the sediments only a short 

 distance away from the edge of the main gabbro mass, and have been traced 

 parallel with this edge for a number of miles. Every observer of the rocks 

 in which these sills lie intercalated states that the rocks are those which show 

 the most extreme effects of the gabbro contact action. They are metamor- 

 phosed so that it is nearly impossible to determine their original character. 

 Evidently they were exposed to the high temperature of the adjacent gab- 

 bro for a long time, and the magma of the sills must have profited by this 

 high temperature of the parent mass of magma and the heated sediments 

 when it was intruded, and cooled more slowly than it otherwise would have 

 done, and much more slowly than the sills farther from the parent mass. 

 Hence the sill rock approaches in its texture much more closely that of 

 the parent mass of gabbro. No reason can be seen why these sills should 

 be connected with the gabbro as gabbro sills and separated from the other 

 sills in their vicinity, and even farther away, Avliich occur under practi- 

 cally identical conditions, and show but relatively unimportant petrographic 

 differences from them. 



5. The sills are very fine grained, almost glassy at the lower and upper sides, 

 even in the thickest sills. The gabbro is not very fine grained at the contact with 

 the Animikie rocks, and even on the edges of the apparent gabbro sills mentioned 

 above, the fineness of grain nowhere approaches that of the edges of the ordinarj^ sills. 



