290 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1930 



igneous rocks are of magmatic origin. Least of all can we proceed 

 from such a supposition in the case of ores among the Lower 

 Archaean rocks. On the contrary we must, as I have shown above, 

 take for granted that sedimentary iron ores not only can but also 

 ought to be f)resent in the Lower Archaean. The metamorphism to 

 which they have been exposed and the admixture of younger igneous 

 matter, they may exhibit, only make the determination of their 

 primary character more difficult. 



The same holds good to a certain extent of the limestones and other 

 para rocks. Besides their chemical character, which indicates a 

 sedimentary differentiation, their occurrence also shows that they 

 belong to a sedimentary series of strata. We find them above the 

 oldest gneisses, that contain neither pyroclastic elements nor rocks 

 formed by sedimentary differentiation (limestones, iron ores, etc.), 

 but they lie below the Upper Archaean with its abundance of fully 

 differentiated sediments. Thus they occur just where the oldest 

 sediments are to be found. 



This is not the place to discuss the primary character of the 

 different gneisses or limestones nor to debate as to which ores may 

 be considered of magmatic and which of sedimentary origin. The 

 reason I have dwelt upon these questions has only been to emphasize 

 the necessity of keeping in sight not only that different possibilities 

 of interpretation exist but also that there are undoubtedly different 

 types of rock, igneous and sedimentary, in the leptite formation. 

 There is often great difficulty in judging their character and it is 

 increased by the fact that the rocks formed by sedimentary differ- 

 entiation, especially limestone and iron ore, frequently appear by 

 subsequent liquefaction in intimate connection with igneous matters. 



As already mentioned, the mineralogically and chemically differ- 

 entiated rocks belong to the second stage in the formation of the 

 eldest subaquatic sediments. The first sedimentation by running 

 and tranquil water consisted of a redeposition of the loose ash- 

 material. When looking for the oldest subaquatic sediments we 

 therefore may expect to find, besides the first sedimentarily differ- 

 entiated ones, also those with the chemical character of the volcanic 

 ash. These rocks ought, as a matter of fact, to form the greater part 

 of the oldest sediments, and we may take it for granted that they 

 originally had the character of tuffs. 



Do then traces of these pyroclastic rocks also exist in the Lower 

 Archaean? Yes, not only traces but large masses. They appear 

 first in the leptite formation and form the bulk of it. It is fully 

 manifested that Swedish leptites as well as the hiilleflinta are mostly 

 and often entirely constituted of volcanic ash. It is also known 

 that these rocks occur almost spontaneously in enormous quantities 

 in the Lower Archaean and likewise that they are accompanied by 



