1893.] evidence for the Recurrence of Ice Ages. Ill 



Greensand a boulder, No. 72, of a green sandstone with shells 

 and masses of phosphate attached and covering striations caused 

 by weathering along lines due to rock structure. But these are 

 in no ways glaciated rocks, and the mode of transport of large 

 masses of rock without the agency of ice we will leave also for 

 separate discussion. 



I have not opened the question whether a glacial deposit may 

 be recognised by other characters such as the arrangement of the 

 material, the shape and condition of the stones irrespective of 

 striation. These are in the present state of the evidence too 

 vague to be profitably discussed, as is also the suggestion of 

 Agassiz that the paucity of life in some formations might be 

 coincident with cold periods. 



xii. Supposed evidence of glacial action in Poikilitic Rocks. 



Of the older ages, such as the incoming of the Jurassic period 

 or of the Carboniferous as recorded in the conglomerates at the base 

 of the New Red or Permian and in those at the base of the Upper 

 Old Red or Devonian there is more to say. In the case of the 

 Lower New Red or Permian some specimens are preserved in the 

 Jermyn Street Museum on which there are stria^, which have 

 been referred to glacial action. These may be divided into two : 

 one represented by a single stone of a greenish colour and flat- 

 tened oval form and covered with strise undistinguishable from 

 those seen on stones from the true glacial drift. This stone 

 must have been placed there for comparison and illustration, and 

 having lost its label have got mixed up at last with those actually 

 found in the Haffield conglomerate. I am well acquainted with 

 that rock, and have no hesitation in saying that the stone I 

 refer to was never in it. The othei^s are specimens with obscure 

 strise such as would be produced by movements in the rock. 

 The general character of this conglomerate and of the fragments 

 included io it may be gathered from an examination of the piece 

 of the Haflfield conglomerate No. 70, and the fragment of close 

 textured mudstone No. 71, with a shiny surface produced by a 

 film of oxide of iron, which occurred in the conglomerate. 



Professor Sellas^ has called attention to this source of error in 

 the case of beds of the same age near Portskewet, Monmouthshire. 

 No. 54 is from this locality. 



There are some accidents which distribute a few glacially 

 striated stones among others that have never been subjected to 

 ice action : for instance. If a clay is broken up into lumps and 

 these are rolled along the shore, pebbles and shells are driven 

 into the sodden exterior which they can help to protect. In this 



1 Qeol. Mng. N. .<? , Vol. viii., 1881, p. 79. 



