mi. J. MORISON — ICE AND ITS WORK. 153 



Greenland does at the present day. AVe nii};ht suppose that there 

 were several such ice-caps ; one for the Highlands of Scotland, one 

 for Northern Ennland, one for Wah's, and so on. 15ut tlicre is 

 other evidence to he found in the low country which proves tliat 

 one single great ice-sheet covered the whole country from the 

 Pcntland Firth in the extreme north of ScotUmd, to at least as far 

 south as the neighbourhood of London, and as far west as the 

 western coast of Irehnid. One proof of this is the occurrence all 

 over the lowlands of Scotland, Ireland, and the greater part of 

 England, of detached blocks of stone or erratic boulders which 

 can only have been brought where we now find them by the 

 agency of ice. So strange sometimes are the positions in which 

 they lie, and so markedly do they often differ in character fi'om the 

 surrounding rocks, that they have been from the earliest ages a 

 source of wonder and amazement. Where did they come from ? 

 There is often no other vestige of naked rock within sight, so they 

 cannot have fallen from any cliif . They cannot have been trans- 

 ported by rivers, as they often stand on the summits of hills. 

 They have not been washed up by the sea or by floods, for some of 

 them are of enormous size, and they often consist of rocks foreign 

 to the neighbourhood, the nearest similar rocks perhaps being fifty 

 or sixty miles away. No conceivable agency but ice, either in the 

 form of a moving ice-sheet, or of icebergs, could have placed them 

 where we now find them. 



Many of these erratic boialders, as they are called, have been 

 found in Hertfordshire, especially in the northern part of the 

 county, but no doubt the greater number of those which once 

 existed have been broken up for road-metal long ago. A very 

 interesting paper on this subject was read before the Society some 

 years ago by Mr. H. G. Fordham, in which he describes nearly 200 

 boulders found in various parts of North Herts, near Ashwell, 

 Hitchin, Royston, Buntingford, and various other places. These 

 boulders are mostly sandstone and limestone of Carboniferous or 

 Jurassic age, but a few consist of granite, dolerite, or other igneous 

 rocks, and two or three of mica schist and gneiss. Mr. Vincent 

 Elsden, in a paper on the microscopical structure of Hertfordshire 

 boulders, gives his opinion that the igneous boulders, which are 

 mostly basic and intermediate, most likely came from the south of 

 Scotland, and that the granitic and gneissose rocks probably had 

 their origin in the Grampians. Mr. E,. T. Andrews, of Hertford, 

 has presented to the British Association Committee for recording 

 erratic blocks a list of 37 so-called boulders. Eighteen of these, 

 however, are of Hertfordshire conglomerate, and therefore have a 

 local origin ; the others, with the exception of one of Carboniferous 

 Limestone found at Ware, are all described as sandstone or grit. 



There is a large block of sandstone standing by the side of the 

 road in Tpper Dagnall Street, St. Albans, about 2i feet in length 

 by 2 feet in breadth, and a foot to 18 inches in thickness, and having 

 externally a rounded concretionary appearance. Mr. Whitaker 

 considers this identical in character with the Sarsen stones of Wilt- 



VOL. VII. — rART V. 



12 



