1853.] BIGSBY GEOLOGY OF QUEBEC. 101 



tached fields to the State of Tennessee. Its breadth is always very 

 considerable. 



The black aluminous azoic limestone of Quebec is of unusual thick- 

 ness for the Hudson River Group. This may be safely stated at 

 2000 feet, and might have been calculated at .5000 feet, but for the 

 disturbances, which make an accurate estimate impossible*. The 

 intercalations, it will be remembered, are few. 



Natural concealments from wide waters, woods, and detritus pre- 

 vent our tracing the extension of this limestone into the adjacent 

 country. 



Usually the Hudson River Group consists of rapidly alternating 

 beds of clay-shale, sandstones, conglomerates, and limestone, from a 

 few inches to 50 feet thick, and occupying wide tracts of country. 

 It is curious to observe that the successively superadded beds have 

 always the same general constitution and chiefly differ in the different 

 state of the pebbles and fragments in the conglomerates and sand- 

 stones, and in the introduction of one or two new ingredients. These 

 appearances seem to indicate a very prolonged epoch, during which 

 a certain set of not very dissimilar acts have been repeated times all 

 but innumerable, and upon the tranquil floor of a shallow sea, which 

 was afterwards elevated. 



The pebbles of the quartzose conglomerates are of the same pecu- 

 liar white, bluish-white, or brown quartz that we see in the Potsdam 

 sandstone near Kingston, Lake Ontario, at Gananoque, and Mont- 

 morenci. 



One of the calcareous conglomerates is evidently derived from the 

 Trenton limestone. 



In so early deposits as the Hudson River Group, it is hard to 

 point out the source of the drab-coloured pebbles of various shades 

 so frequent at Lauzon and elsewhere. According to our present 

 knowledge, we have only to look to the Birds' Eye and Calciferous 

 limestones as their parent rocks ; with these, however, I am but little 

 acquainted. Nor do I understand clearly how the Hudson River 

 Group came to be conformable to the gneiss of this district, with two 

 unconformable strata of great thickness, and widely different, in- 

 terposed. 



* This limestone, it will be remembered, occupies the whole breadth of land 

 from the lliver St. Lawrence across to the River St. Charles, in front of the For- 

 tifications looking westerly, a distance of 1837 yards. 



