888 
Proceedings of the Royal Society 
III, Shapes of Boidders. 
Two classes may be specified — (1) angular and rough, (2) rounded 
and smooth, on. the surface. 
In all the Scotch counties, both of these classes exist]— with this 
distinction, that the second class are generally embedded in drift, 
whilst the first are mostly, at all events, now, on the surface of 
the district {Abstract, pp. 849, 850). 
If, as may be assumed, the erratic blocks referred to in the Com- 
mittee’s Eeports were originally fragments from rocks in situ, then 
it is ]3i’obable that the most rounded are those which have undergone 
most wear and tear ” by transportation. 
Boulders of both classes, have often a long and a short axis 
smooth boulders more frequently so, than others. The latter are 
also frequently Pear-shaped,^' indicating that one end has probably 
undergone more friction than the opposite end. See, as an example, 
Dana, boulder,” on p. 781 of Abstract, 
In such cases it has also been observed that when one end is 
smooth and sharp-pointed, the opposite end is generally square or 
rough. 
IV. Partieidar Alarhings on Boulders. 
On some Boulders there are occasionally grooves, ruts, strise, 
and scratches upon their surface when smooth. 
The incisions generally form lines approximately parallel with the 
longer axis of the boulder. They may occur on one or more of the 
sides, i,e,, along the upper, lateral, and under surfaces. 
Examples of marks on the under surface will be seen by referring to 
the Abstract, p. 769 {Aberd.een)\ p, 808 {lonai) ; p. 845 {Tynecastle) 
and p. 847 {Alnioick Hill), 
It has been thought, that from a close examination of ruts 
and striae, whether on boulders or on rocks, the direction of 
the striating agent can be inferred by observing at which end 
the striae have been most deeply cut. In multitudes of cases 
it has been observed, that the striae are more deeply cut at one 
end, whilst towards the other end they gradually thin away and 
disappear. In explanation of this fact, it is suggested that hard 
pebbles or stones, acting as incising tools, would, in advancing along 
