288 DH. A. GILLIGAN ON THE PETROGRAPHY OF [vol. lxXV, 



been a continental land through elevation of the Carboniferous 

 group, until the northern extension of the Liassic seas to these high latitudes. 

 The complete absence (so far as we know) of Permian and Triassic strata in 

 the Parry or Northern archipelago goes far to confirm this. Not a trace of 

 any organic remains younger than the Carboniferous Limestone and older 

 than the Miocene has occurred to Feilden and Hart, or to any of the other 

 explorers during their researches in these high latitudes.' 



This uplift of the northern area at the close of Carboniferous 

 Limestone times premised by Etheridge is fully corroborative of 

 the general uplift of continental dimensions which is certainly 

 demanded by the Millstone Grit Series, and therefore the closing 

 of the gulfs or fiords would naturally follow. That the same 

 southward recession of the shore-line took place on the western side 

 of the Atlantic, in America, is quite clear, for there the Mississip- 

 pian or Lower Carboniferous is overlain by the Pottsville Con- 

 glomerate (the deposit which is homotaxial with our Millstone 

 Grit) in the east, which grades westwards and south-westwards into 

 sandstones, and with local conglomeratic phases the sandstone is 

 found over the interior of the continent. This conglomerate and 

 sandstone is composed of material derived from the Archaean 

 which lay to the north. It is thought that the conglomeratic beds 

 of the east represent the earlier stages of the epoch, and that, as 

 the period of deposition was prolonged, the detrital material spread 

 westwards and southwards. The analogy, therefore, with Britain 

 would seem to be complete. Chamberlin & Salisbury have 

 estimated that the sea was excluded from an area of about 

 20,000,000 square miles at the close of the Carboniferous Lime- 

 stone period. 1 



The physical conditions of the Millstone Grit period with the 

 high land to the north, probably forming a continuous belt for 

 some thousands of miles from Europe across the Atlantic, and 

 embracing part of Northern America, with deltas encroaching upon 

 the ocean to the south, would determine a climate of a monsoon 

 type. The warm southerly winds would bear an abundance of 

 water- vapour to be condensed on ascending the slopes of the high 

 land and capable of feeding the great rivers of this period, which 

 in their rejuvenation would soon clear off all the rotted rock left 

 by the leaching processes of the preceding period. The relief of 

 the land previously discussed may have been such as to nourish an 

 icefield and its attendant glacial phenomena ; but the snow-line 

 would necessarily be at a fairly-high altitude, on account of the 

 proximity of the warm southern ocean and the warm winds which 

 would blow therefrom. 



Weathering of the rocks would be chiefly by mechanical means, 

 in which chemical action played a minor part. As was first pointed 

 out by the geologists of the Indian Survey, 2 it is possible to 

 determine whether mechanical or chemical agents have been the 



1 T. C. Chamberlin & E. D. Salisbury, ' Geology : Earth History ' vol. ii 

 (1906) pp. 658-59. 



2 H. B. Medlicott & W. T. Blanford, ' Manual of the Geology of India ' 

 2nd ed. by R. D. Oldham (1893) p. 201. 



