250 The American Geologist. ^i»"' ^^o^- 
(2.) The siliceous rock resembling the last but evidently- 
resulting- from oceanic chemical precipitation, forming long 
parallel bands intersheeted with greenstone debris and with 
other sediments, and grading into ordinary slate, or graywacke 
or into a siliceous greenstone. These bands are not wavy nor 
curled, nor structurally suggestive of lava flows. 
(3.) A detrital debris resulting from the destruction of 
either of the foregoing. When compacted, and especially when- 
metamorphosed, such secondary jasper bands are sometimes 
hard to distinguish from primary jaspilyte. 
All of these forms of jaspilyte may be so charged with 
iron oxide as to constitute iron ore. 
(10.) After prolonged field examination the Minnesota 
Survey reached the conclusion that the granites of the Archean 
grade into gneiss, the gneiss into micaceous gneiss and mica 
schist, and finally into less and less metamorphic rocks that 
show a plain fragmental structure and sedimentary origin. 
There was found no exception amongst the Archean granites. 
The granites are of two dates of formation, one at the close 
of the Lower Keewatin and one at the close, or after the close, 
of the Upper Keewatin. A later granite, associated with the 
gabbro, and grading into it, is of the Keweenawan, and another 
is still later. Hence it was inferred that the Archean granites 
did not spring from a deep source, but are a surface product 
of metamorphism carried to the extreme of fusion, on clastic 
materials that were later than the basal greenstones. Adven- 
titiously they form intrusions in some of the later (and espec- 
ially into the clastic) greenstones, but they are not known 
to penetrate the oldest greenstones. Tentatively the alkaline 
and the acid siliceous elements in these early sediments were 
supposed to have, been derived from the atmosphere, as the 
basal crust could not have afforded them. 
(11.) In the same manner the gabbro, which becomes acid 
and grades into syenyte, was derived from the metamorphism 
and fusion of the greenstones with their elastic variations. 
Diabase was found to pass insensibly into gabbro, but on the 
other hand it is also certain that it was the original form of all 
the igneous greenstones, and that it must have had a deep 
Sicated source, and has still a deep seated source. 
