660 THE HURONIAN AND LAURENTIAN PERIODS. 



B Its opening, if we may judge by the lowest member known, was 

 marked by the accumulation of littoral sediment. To this succeeded 

 an epoch when igneous eruptions commingled molten matter, scoria, 

 and fragments of rock with the fine mud resulting from the wearing 

 of the Azoic continent. After an interval of time, during which the 

 arenaceous shales of No. 3 were formed, these conditions were again 

 repeated in a still greater accumulation of volcanic ashes, tufa, etc., 

 which, as the pre-existing land sank beneath the waters, spread as a 

 thin deposit further west. 



" The whole was eventually covered by the red and purple sedi- 

 ments of the Upper Division, which are more uniformly distributed, 

 and are conformably surmounted by the lowermost strata of the Lower 

 Silurian formation, thus becoming, like the Cambrian of Britain, the 

 ' basement segments of the Silurian system.' And although Professor 

 J. D. Dana classes these fundamental rocks of the Palaeozoic series as 

 Azoic, he remarks, that ' should the Huronian rocks be hereafter 

 found to contain any fossils, they will form the first member of the 

 Silurian.' 



" In general characters there is a remarkably close resemblance 

 between this formation and the Huronian of Canada, notwithstanding 

 the wide extent of country which intervenes. Both are largely com- 

 posed of erupted materials, diorites, tufas, and volcanic mud : hard- 

 ness, and obscurity in the lamination of the slates is a feature in 

 common ; and here, as in Canada, slate conglomerates may be seen of 

 a texture so compact and uniform that the inclosed masses are dis- 

 tinguishable only by a difference of colour." 



The structure and composition of the series are thus given by Mr 

 Matthew, in ascending order : — 



" Lower Division. 



"1. Coarse red conglomerate (with an abundance of quartz pebbles) 

 and red sandy shale. 



"2. Dark porphyritic slates and trap, with slate conglomerate, trap- 

 ash, and tufa. 



" 3. Gray and ferruginous arenaceous shale and sandstone, becoming, 

 when altered, a laminated compact felspar or felspathic quartzite. 



" 4. Pale-green (weathering gray) slate, stratification very obscure 

 [apparently an indurated volcanic ash], with slate conglomerate, ash- 

 beds, and tufa. 



" Upper Division. 



" 5. Red and gray conglomerate and red shale. Red and purple 

 grit and sandstone. 



. 







