PORTLAND ROCKS OF ENGLAND. 223 



Conclusions. 



Accepting the above results we can now give some account of the 

 Portlandian episode in England. Remembering that little trace of 

 any such episode can be detected either at Speeton or beneath Sussex, 

 we learn that the elevation which introduced the circumstances 

 capable of j^ielding sandy and calcareous deposits took place spora- 

 dically. We also know from the borings in London and at Ware 

 that 30 miles from their most easterly outcrop the Portland rocks 

 are not to be traced ; but on this much reliance cannot be placed, 

 for they may have been removed by precretaceous denudation. So 

 far as our knowledge at present extends, the earliest rise took place 

 along the main axis between Boulogne and the Mendips, and gave 

 the peculiar character to the lower part of the beds west of Tisbury, 

 while the Portland area was scarcely affected by it, and the clay to 

 the north was rendered more sandy. Further physical changes of 

 a nature unknown to us brought about the denudation of some 

 sandy rocks and developed the great sandbacks, which pass from 

 Swindon to Shotover and die away to the north-east, and which 

 diminish from St. Alban's Head towards Portland and TJpway and 

 are not to be recognized in the Boulogne area. On the north side 

 of the elevation these sands are partly glauconitic, the amount of 

 green matter becoming greatest as they thin away to the north ; 

 while on the south side I have failed to find any glauconite, and on 

 the line itself at Tisbury the slight thickness of sand exposed is 

 brown as from the presence of a peroxide of iron. These peculiarities 

 point to a different history of the deposits. The dark glauconitic 

 (? all) grains are certainly not due to Foraminifera directly, calca- 

 reous specimens unfilled with dark matrix being present with them, 

 and they having no shape derivable from such a source. Many of 

 the dark-coloured stones are not glauconitic but Lydian stones. 



Slight succeeding oscillations introduced the clay of Swindon and 

 Portland and the upper bed of sand in Oxon and Bucks ; but the 

 seas were henceforth clear of these, and Mollusca flourished in one 

 place to have their broken remains deposited in another, either of 

 recognizable size or ground to the finest powder to form the chalky 

 rocks. In one of these oscillations in the northern district a colony 

 of Mollusca which had flourished and died and had their remains 

 compacted to a rock, was broken up and the debris deposited again 

 as the rubbly rock of Buckinghamshire. 



The shallower sea to the north was soon, nearly filled, while the 

 axis of elevation travelled northwards, so that the former became 

 an inland lake, and the accumulations at Swindon brought the sea- 

 bottom so near the margin of the land that irregular deposits took 

 place in it, though still marine ones. Shortly, however, this spot 

 became dry land, suffering minor oscillations, marked by the erosion 

 of bed after bed, the accumulation of unsorted debris, and the 

 deposition of freshwater limestones. Meanwhile the sea to the south 

 was still open, chalky and flinty masses of considerable thickness 

 continued to be formed, varied at last by rougher deposits formed in 

 more troubled water, till finally the last of the marine animals 



