PLATE LXXVIII. 



THE GABBRO LAKE PLATE OF THE MESABI IRON RANGE, 1899. U. S. GRANT. 



In the area of this plate are important and interesting exposures of the Archean, 

 including greenstone, with jaspilyte, mica schist, dioryte and intrusive granite and 

 syenyte. 



The field notes and descriptions made by the writer (1886), quoted by Dr. Grant 



* 



in the text of volume iv, represent the greenstone as mainly igneous about Fall lake 

 and Garden lake, but later investigation of the rock samples collected has shown 

 that, so far as known, the greenstone of that region is all, or nearly all, of clastic 

 origin. The succession of parts described at Garden lake, illustrated by rocks Nos. 

 1017 to 1023, was responsible for the inference by the writer that the schist gradu- 

 ated into a massive crystalline rock, i. e., from No. 1017 to 1023. The inference is 

 correct so far as concerns the graduation, but the rock No. 1022, from which the 

 schist was supposed to have been derived, is not a massive crystalline. It is simply 

 a more coarse one than the schist, and both are clastic, sheared, basic, consolidated 

 Keewatin sediments, the gradation being one of varying sedimentation. This early 

 error was corrected when the rock samples were finally examined (volume v, p. 633). 

 It is in rock of this kind, but usually more plainly of fragmental origin and 

 even of sedimentary structure, that occur the jaspilyte ore bodies of the Vermilion 

 range. Generally the main structure and direction of trend of the ore bodies agree 

 with the same of the greenstone, and there is sometimes a sedimentary alternation 

 of jaspilitic rock with green schist, showing a subordination of the jaspilitic rock to 

 sedimentary forces. But besides this structure, which is sometimes only a marginal 

 feature of the ore lenses, though it sometimes characterizes the whole mass, the jas- 

 pilyte manifests a different, much curling, structure, which runs in defiance of the 

 general direction and turns upon and within itself. Such curling jaspilyte has never 

 been seen to contain bands of sedimentary green schist, or other fragmental mate- 

 rials, and seems not amenable to the sedimentary theory. It is interpreted by the 

 hypothesis that originally this rock was a basic obsidian, erupted in submarine con- 

 ditions, and, being in a noncrystalline condition, was silicified by the deposition of 

 silica from the oceanic waters, the silica preserving, as in fossil wood, the structures 

 of the original. Cotemporary with this silicification of the igneous mass other chem- 

 ically precipitated silica was arranged conformably in a sedimentary banding and 

 sometimes in alternation, or closely mingled with all the products of detrital action 

 that could be furnished by the physical environment. Thus are produced two forms 



