148 B. Smith—-Ball or Pillow-form Rocks. 
a pillow-form (Fig. 1), ball-like, or semi-spheroidal habit. The 
spherical contours are usually developed on the undersides of 
projecting ledges of weathered sandstone as rounded lobes and curved 
surfaces, whilst occasionally they occur on the upper surfaces as well, 
when the rock becomes still more pillow-form in appearance. The 
spheroids are hardly ever completely developed. If they were they 
would average about 1 foot in diameter, with a maximum of 2 ft. 6in. 
The pillow-form shapes are sometimes arranged parallel to the bed,’ 
but are frequently inclined. They are often twice as long as thick, 
and average from one to two feet in length. 
Externally the shapes of these masses recalled those of pillow- 
lavas, or the spheroidal weathering of dolerite, and since some of the 
pillows contained much felspathic material and some of the spheroids 
appeared to be due to weathering, their occurrence was sufficiently 
interesting to warrant further investigation, which showed that they 
were of two kinds. 
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Fig. 2.—Shell-jointing in sandstone. 
Two Types of Structure. 
1. Structures due to jointing and weathering (shell-jointing). 
2. Pillow-form or ball structures due to internal build. 
1. Where the rocks are evenly bedded and well jointed the corners 
of roughly rectangular masses, blocked out by joints and planes of 
stratification, are liable to shell off in layers roughly concentric with 
the central portions of the blocks. As in dolerite the shell structure 
may be due primarily to shrinkage. In undoubted cases (Fig. 2 
where this shelling off takes place ' the fracture was unrelated to the 
lamination, which runs normally through the pillow. The exposed 
top corners of a block are usually shelled off first, unless the lower 
corners project prominently from the underlying beds. On 
hammering at such a pillow the concentric weathered shells may be 
knocked off in turn until only the tough unweathered core remains. 
2. In other cases, if we hammer at a pillow or spheroidal mass we 
discover that its internal structure may correspond with its curved 
face; that is to say, the lamine of sandstone are curved in conformity 
with the outer curve of the pillow. A good example occurs in an 
old quarry in Llwyn Bryn Dinas Wood, about one mile west of 
Llangedwyn. Some of the sandstone strata are evenly bedded and 
1 For this reason the rock is unsuitable for use in the exposed corners and 
angles of buildings. It is known locally as ‘ grinsel’. 
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