THE FIRST liAINS AND TIIEIK GEOLOGICAL 

 SIGNIFICANCE ' 



By AssAU Uauuinu 



In stiulying the sedimentary rooks one has to face problems of the 

 most varying nature. I have before had occasion to discuss several 

 of these, especially those related to the formation of conglomerates. 

 In connection with my investigations on these rocks the following 

 (piestions also arose: Which are the oldest sediments on the earth? 

 Where do they occur? When were they formed? How are they 

 constituted? As will be seen in the following these questions also 

 involved others, and especially one of them — When did the first 

 rains fall on our earth and what was their effect? — must stimulate 

 our fancy. I shall here only present a few reflections, to which 

 these and kindred questions have given occasion. 



THE CONGLOMERATES INDICATE OLDER SEDIMENTARY SERIES OP 



STRATA 



On examination of a conglomerate, pebbles of older sedimentary 

 rocks are often found. These pebbles are naturally of a particular 

 interest in cases where they are the only remains of a series of strata 

 otherwise broken down. 



The Lower Cajnbrian sandstones and the basal conglomerates en- 

 closed in them are the oldest known undoubtedly marine deposits 

 in Sweden. They were formed on the shore of the Cambrian sea 

 during its first transgression. In these conglomerates, however, 

 pebbles of sedimentary rocl«, derived from earlier deposited strata, 

 also occur. There is no reason for astonishment at the presence 

 of these pebbles, as from several parts of the country we are aware 

 of sedimentary series of strata of pre-Cambrian age, which are 

 formed in continental basins. 



On turning to the pre-Cambrian series of strata, we also find in 

 these only slightly metamorphosed sandstones and shales. But the 

 rocks in the said series are for the most part more or less meta- 

 morphic, transformed into quartzites, phyllites, mica-shists, and 



1 From a lecture delivered before the K. Fyalogr, Siillsk. at Lund on Mur. U. 1927. 

 Reprinted by |)<?ruilsslon from Geologlska Forcnlngeus i Stockholm Forhandlingar, Janu- 

 ary-February, 19-9 



285 



