1873.] MACKINTOSH — REMARKABLE BOULDERS. 359 



proportion of large blocks ! The largest I saw during this visit was 

 nearly 5 feet by 3| x 3, but I was informed that several much 

 larger blocks had been buried at Trescott, and that others might be 

 seen in Trysull parish. I believe that within an area about three 

 miles in diameter there must be many hundreds of granite blocks 

 upwards of 3 feet, and many thousands upwards of 1|- foot in ave- 

 rage dimensions. Many of them may be seen by the sides of hedges 

 and lanes ; and they form the foundations of many walls. In Che- 

 shire and Lancashire most of the boulders are either rounded or 

 glaciated ; but, though at Trescott I saw two or three boulders of a 

 fine-grained rock polished and striated (and during my first visit 

 one block of granite), on this occasion I could detect no strise on any 

 granite blocks distinct from marks left by the plough and harrow, 

 while very nearly all of them were strikingly angular. I believe 

 that they were not transported along with the rounded and glaciated 

 boulders found in and at the base of the Lower brown clay of more 

 northerly districts, but at a later period, and that nothing short of 

 large icebergs at a time when the sea was deep enough to float them 

 without grounding until they struck on an elevated part of the now 

 desiccated sea-bed west of Wolverhampton, will account for the vast 

 array of large blocks which are there crowded within so small a 

 compass *. 



EXPLANATION OF MAP, PLATE XIII. 



This map is principally intended to show the character, positions, sources, 

 and probable routes of a number of the more remarkable boulders of the N.W. 

 of England and N.E. of Wales. To prevent overcrowding, extensive groups of 

 boulders are frequently represented by a few dots, and many boulders I have 

 not particularly examined are omitted. Only lines pointing to the sources of 

 the granites have been inserted. With the exception of the eastern part of the 

 Shapfell dispersion, the map is founded on the results of personal observation. 



Discussion. 



Mr. Tiddemak pointed out that, although the general course of a 

 boulder might be represented by a straight line drawn from its 

 native rock to its last resting-place, this did not necessarily coin- 

 cide with the direction of any of the stages of its migration. Besides 

 land-ice, these may have been effected by river-action, and possibly 

 by floating ice during a submergence before the time of the ice- 

 sheet, and certainly by both those agents after. 



Mr. Hicks called attention to some of the boulders to be seen in 

 the south of Wales, which have been striated by the action of the 



* Dr. Buckland in his 'Keliquirc Dilnviance' (1824) referred to large boulders 

 between Dudley and Bridgenorth, which are probably a part of the great group 

 under consideration, and regarded them, on the authority of Greenough's Map 

 of England, as Kavenglass (Eskdalc) granite. Sir P. I. Murchison, in his ' Silu- 

 rian System,' mentioned the Trescott and Trysull boulders, and, in his address 

 to the Geological section of the Birmingham meeting of the British Association 

 (1865), correctly assigned to them a Scottish parentage, and spoke of them as 

 having been transported by icebergs. It may be necessary to add that, though 

 the Trescott and Trysull boulders form the most remarkable group, there are 

 many boidders in other parts of Staffordshire. 



