THE FIT^ST RAINS — HADDING 289 



WHERE ARE THE OLDEST SEDIMENTARY ROCKS TO BE FOUND? 



As already preliminarily mentioned, the Loayoi- Cambrian con- 

 <rlomerates contain pebbles from pre-Cambrian sediments, and the 

 con<rlomerates in the Upper Archaean contain pebbles from earlier 

 deposited sedimentary series of strata. In this way we can immedi- 

 ately follow the formation of sediments back to the Lower Archaean. 



If we look for sedimentary rocks in the Lower Archaean, we must 

 however understand (liat they can not appear in their orij^inal form. 

 No part of the earth's crust has undergone so many and so thor- 

 oughly metamorphic processes as the oldest one, and in no other part 

 have the rocks been so mixed with injected igneous matter. 



Thus we can only expect to find the oldest sediments in a highly 

 metamorphic shape and probably strongly broken up by younger 

 igneous rocks. 



In the coui-se of time a great part of these rocks must have been 

 destroyed by erosion or concealed below younger strata and partly 

 also pressed down to such a level that they could partake in a mag- 

 matic circulation. What now lies denuded can therefore only be a 

 very small part of what once was formed. AVc turn to the oldest 

 Archaean regions in search of it. 



The two regions to be thought of next are the North American 

 and Fenno-Scandian. Both are well known by the ardent studies 

 pursued for many years by a very great number of geologists, 

 amongst whom we find several of the leading petrographers and 

 geologists of the present and the past generation. Thanks to the 

 works of these students we are at the present day very well able to 

 survey the origin of the Archaean rocks. We shall only turn to the 

 Swedish region. Is there then any cause to suspect that some of its 

 rocks belong to the oldest sediments on the earth? My answer will 

 without hesitation be affirmative, and I am persuaded that every- 

 body, who knows the Swedish Archaean rocks, and who approves of 

 ihe train of thouglit I have followed above, must answer in the same 

 manner. And moreover, everybody must come to the same conclu- 

 sion. The Swedish pyroclastic leptite formation contains the oldest 

 .subaquatic sediments on the earth and the limestones and iron-ores 

 connected with this formation bear evidence of the first chemical 

 weathering and differentiation of sediments. We shall discuss the 

 premises of this conclusion. 



No doubt, many will immediately object: "The iron ores in the 

 Swedish leptite formation are igneous." Nobody can deny that ore- 

 minerals occur primarily in the igneous rocks and certainly nobody 

 can doubt that iron ores also may occur as products of magmatic 

 differentiation. We have sufficiently good evidence of this in many 

 quarters. But it does not imply that all ores occurring together with 



