304 Seeley — Rock of the Cambridge Greensand. 



any fragments from the country of ancient rocks whicli was then 

 being worn away. This probability becomes almost certainty when 

 a comparison is made of the thickness of the Upper Greensand in 

 different sections. Stated in feet, this is : Ely 1, Cambridge 1, Ash- 

 well 1, Totternhoe 7 to 10, Merstham 25, Folkstone 30, Man o' War 

 Cove 35, Wroughton 40, Woolstone 60, Swindon 70, Lulworth 76, 

 Hampshire 80, Eidge 100, Culham 100, Didcot 120, Devizes 138, 

 Isle of Wight 150. Everywhere it has been noticed to get thinner 

 to the East and Norih-East ; thicker to the West and South-West. 

 And this I suppose to be owing partly to a bottom gradually shelv- 

 ing deeper to the South, partly to a slight prevailing current piling 

 the sand in the hollow, but chiefly to the Southern area being nearer 

 to the old Paleeozoic and Plutonic Rocks whence the sand came ; — ^to 

 such rocks in fact as actually occur on the opposite region of France, 

 and probably form the adjacent floor of the Channel, which the 

 Shanklin Sands prove were denuded in a previous age, and which 

 were probably the origin, of the sand of the Portland Eock. Other 

 granitic centres may have furnished quartz grains as well. But this 

 French plutonic country could have every winter sent out a freight of 

 boulders, such as those sharp angular blocks which ice, or trees, or 

 a continuous shore could have landed where now found. 



Hence the Cambridge area was a long, low coast where few Fora- 

 minifera lived, and only stray boulders were occasionally carried, 

 while the finely comminuted material was moved away into more 

 open seas to the South ; while to the North, in the Hunston Eock, 

 scarcely a boulder ever found its way ; and the deposit, becoming 

 eminently calcareous, gradually thickens. 



The next feature, noteworthy in the rock, is the green mineral 

 grains colouring the marl, and sometimes also the phosphatic 

 nodules. One very important thing about the glauconite, as the 

 green mineral is named, is its vast geographical range over Europe, 

 and through much of America. And scarcely less remarkable is 

 the association in which it occurs. In England the oldest bed, in 

 which it is characteristic, is the Sand of the Portland Eock ; then 

 it occurs in amazing quantities in the Shanklin Sands, and again in 

 the Upper Green Sand, dying away in the Chalk, to reappear in the 

 same locality with the Sands of Bracklesham, and other Tertiary 

 strata. Thus, while the wide-spread geographical range suggestively 

 points to an origin as wide-spread as the sea, the geological range, 

 limited for the most part to an association with sands, all derived 

 from the same parent rocks, seems more decidedly to indicate an 

 origin from local causes. Professor Eogers would have attributed 

 it to a former Gulf-stream ; but, unfortunately, there are not suffi- 

 cient data to tell what causes are producing the glauconite which is 

 now forming along the American Coast. 



But it is to be questioned whether the different analyses of glau- 

 conite given by Turner, Berthier, Dana, Eogers, Van der Marck, etc., 

 do not really indicate different minerals. All the grains are amor- 

 phous, and on being powdered, yield a bright gi-een colour. And it 

 deserves note that some analyses of glauconite are singularly 



