PRE-CAMBRIAN NORTH AMERICAN LITERA TURE J 5 I 



and the gabbro. This rock, has all the characteristics of an eruptive 

 younger than the gabbro. The coarse-grained rocks between the gabbro 

 and the keratophyre are intermediate in character between the two, and 

 grade into them. They are therefore regarded as a contact product 

 formed by the intermingling of the gabbro and keratophyre magmas. 

 The keratophyre also apparently grades into the Animikie slates and 

 (juartzites, there being three zones showing different grades of altera- 

 tion of the sedimentary rocks, due to the contact with the igneous 

 rock. 



From the peculiar relations it is regarded as probable that the 

 keratophyre is of contact origin ; that is, it was produced by the 

 fusion of the slates and quartzites of the Animikie through the action 

 upon them of the gabbro. The magma thus formed then acted in all 

 respects like any intrusive magma. It penetrated the surrounding 

 rocks in the form of dikes, and solidified as a soda-granite under cer- 

 tain circumstances, and under others as a quartz keratophyre. Cut- 

 ting all of the previously mentioned rocks are diabase dikes. 



Bayley' gives a detailed petrographical study of the basic, massive 

 rocks of the Lake Superior region and especially of the great gabbro 

 of northeastern Minnesota. The normal phase of the gabbro is 

 found to have a typical granitic structure and to differ essentially 

 from all of the basic intrusive rocks of the Animikie series and from 

 all other Keweenawan basic rocks, none of which have a distinctly 

 granitic structure. Upon the border of the main mass of gabbro are 

 peculiar rocks which are interlaminated with quartzose bands. These 

 are shown to be but peripheral phases of the gabbro. It is concluded 

 that further field work will probably show that the gabbro is either a 

 batholite, well toward the base of the Keweenawan series, or that it is 

 a eroded mass upon top of which the later Keweenawan beds have 

 been deposited. 



Elftman"" finds that the great gabbro of northeastern Minnesota 

 has a rude arrangement of the rock in parallel layers similar to the 



' The Basic Massive Rocks of the Lake Superior Region, by W. S. Bayley. 

 JOURN. OF GeOL., Vol. I, 1893, pp. 433-456, 587-596, 688-716; Vol. II, 1894, pp. 



814-825; Vol. Ill, 1895, pp. 1-20. 



^ Notes upon the Bedded and Banded Structures of the Gabbro and upon an 

 Area of Troctolyte, by A. H. Elftman, 23d Ann. Rep. Geol. and Nat. Hist. Sur. of 

 Minn, for 1894, part XII, 1895, pp. 224-230. 



