GEOLOGY OF NORTH ATLANTIC — GILLIGAN 217 



times. If our paleographic maps are even approximately correct, 

 the area occupied was between 80,000 and 100,000 square miles, and 

 if they had, let us say, only an average thickness of half a mile, then 

 on the lowest estimate we get a total volume of 40,000 cubic miles. 

 The Mississippi to-day brings down about the twentieth of a cubic 

 mile of sediment yearly. I have previously shown reason to believe 

 that the bulk of the clastic material in tlie Carboniferous is of gran- 

 ite type and came from the northerly direction. 



The Mississippian (Lower Carboniferous) is extensively developed 

 (as its name implies) in the Mississippi region of Central America, 

 southwest of the Great Lakes. When the clastic deposits were en- 

 croaching from the east, these lower beds were upraised and denuded 

 so that the Upper Carboniferous or Pennsylvanian rests uncon- 

 f ormably upon the several members, not only of the Mississippian, but 

 also of the Older Paleozoic formations. The Pottsville conglomerate 

 (the equivalent of our Millstojie grit) and the succeeding Carbon- 

 iferous rocks in the Appalachian region, are regarded by American 

 geologists as having been brought into the great Appalachian Geo- 

 syncline from an easterly and southeasterly direction, overlapping 

 one another away from the source of supply. The Lower Coal 

 Bearing or Productive Measures overlap the Pottsville to the west 

 and are most extensively developed in the eastern region. As in 

 Britain, so in America; after the Pottsville conglomerate period 

 there was a reduction in the strength of the currents, so that the 

 lowest beds of the Productive Measures are deposited in the east 

 only. All the higher series (Alleghany and others) were also derived 

 from what the American geologists call " Appalachia," which was a 

 land mass off the east coast of America. The folding of the geo- 

 syncline and uplift of the Alleghan}^ Mountains proceeded pari 

 passu with the depression of the old land from which the sediments 

 had been derived and the coarsest eastern deposits were at the same 

 time removed. Carboniferous rocks of the clastic type are also well 

 developed in Nova Scotia, Newfoundland, etc. 



I had the opportunity some years ago of examining many speci- 

 mens of carboniferous rocks from Newfoundland, through the cour- 

 tesy of Mr, Landell Mills. One interesting bed known as the 

 Huniber grit (a very appropriate name) bears a striking resemblance 

 to such a rock as the Eough Eock, say, of Horsforth near Leeds, 

 Yorkshire. Neither in hand specimen nor under the microscope 

 could it be distinguished from it, and it yielded a similar suite of 

 heavy minerals. Especially noteworthy is the feldspar, which is 

 dominantly microcline, microcline-microperthite and oligoclase. 



The physical conditions appear to have been almost identical on 

 each side of the North Atlantic during the Carboniferous period. 



