160 NEOZOIC TIME. [CHAP. IX, 



it must have emptied itself into one of the two marine 

 gulfs, and must have formed a great delta, so that we 

 should have expected the occurrence of a thick mass of 

 estuarine Portland-Purbeck deposits either in one basin or 

 the other, but there is no sign of such a formation ever 

 having existed in either. 



K , however, a large lake existed on the site of the Irish 

 Sea, as indicated in Plate VIII., the detritus brought down 

 by the mountain streams would find a resting place, and 

 the excurrent river would be as clear as the Rhone when 

 it leaves the Lake of Geneva. Such a river, flowing 

 southwards and not receiving any important affluents, 

 might enter the southern gulf without giving rise to 

 any extensive estuarine or deltaic deposits, or leaving 

 any larger record of its existence than such a channel 

 and break in the marine sequence as is found at Swindon. 

 The Swindon section seems to indicate the presence of 

 a river of some size, and if the north-western river did 

 have the course above suggested this may have been its 

 debouchure. 



Besides this river the only other streams, so far as we 

 can see, that could enter the southern or Purbeck basin 

 would be those draining the country to the west and south. 

 Now in Portland times the influence of these rivers does 

 not seem to have been great, and possibly they then carried 

 more material in solution than in mechanical suspension. 

 Even the subsequently-formed Purbeck beds of Dorset 

 are less fluviatile than lacustrine in character, and Mr. 

 Meyer has indeed argued for their purely lacustrine origin. 

 He goes so far as to speak of the Purbeck basin being 

 " probably from its commencement rather that of a lake, a 

 series of lagoons, or even of an inland sea, than of an 

 estuary in the ordinary meaning of the word." ! This. 



1 " Quart. Journ. Geol. Soc.," vol. xxviii. p. 245. 



