154 THE VERMILION IRON-BEARING DISTRICT. 



The above rocks, when examined in the laboratory, give no ckie to the 

 character of the rocks from which they were derived, if we except the case 

 of the specimen containing the amygdules. Classified according to their 

 mineralogic composition we would call them chlorite-, amphibole-, and 

 biotite- schists, and gneisses. When studied in the field, however, their 

 intimate relations with the greenstones and the gradations observed between 

 the schists and the massive greenstones prove conclusively that they have 

 been derived from rocks similar to those from which the massive greenstones 

 that now predominate throughout the district have been derived — in other 

 words, from dolerites, basalts, and andesites. 



ORIGI]Sr OF ELY GREENSTONE. 



There can certainly be no reasonable doubt in the mind of the reader 

 as to the original character of the rocks described as constituting the Ely 

 greenstone of the Vermilion district of Minnesota. The various textures 

 and structures that the rocks possess are such as are present only in igneous 

 rocks. However, even though it may be conceded that the greenstone 

 formation is of igneous origin, there remain still the further queries: Are 

 the rocks constituting it intrusive or effusive in their character, or are 

 rocks of both of these modes of foi-mation present in the complex? 

 Furthermore, if both occur, which mode of origin is the predominant one? 

 These points may well be discussed here. To the first query the answer 

 must be given that the observations recorded show that both kinds of 

 rocks — both effusive and intrusive — are present. The answer to the second 

 query, as to which of these predominates, can be given only with some 

 doubt, as it is very difficult to make a quantitative estimate of the areal 

 distribution of rocks that are so similar in character. From personal 

 observations, however, the writer has been impressed by the very wide 

 distribution of the greenstones that 2D0ssess characters 'indicative of 

 effusion. This has led him to place the greenstones with the surface flows; 

 but the reader must be cautioned to include under the term surface flows 

 those which may have been poured out under water — submarine flows — 

 and which were thus, perhaps, under relatively high pressure, as well as 

 those that reached the surface of the land. In either case the mode of 

 origin outlined for many of these greenstones postulates a su.rface upon which 

 they could rest. Hence, if interpreted in the sti'ictest sense, they do not 

 actually represent the original crust of the earth, as Winchell considers them 



