_ 
B. Smith—Ball or Pillow-form Rocks. 147 
1. Llandilo Sandstones.—In the west thick lenticles of sandstone, 
or masses of alternating lenticles and shales, occur at infrequent 
intervals, as at Llanrhaiadr, where one bed is as much as twenty feet 
in thickness. Farther east the beds of sandstone appear more 
frequently, and are often thicker than those in the overlying Bala, 
and sometimes of coarser type. They are well-developed near Moel 
y Gwelltyn, Gyrn Moelfre, Craig-yr-hwch, and Foel Rhiwlas. 
The sandstones are blue laminated gritty quartz-felspathic and 
slightly calcareous rocks, often weathering like an ashy sediment; 
whilst some beds are true ‘ashy sandstones’. They contain pellets 
and pebbles of shale and mudstone, and in weathered specimens show 
brown or yellowish-red earthy inclusions. Like the Bala sandstones 
they contain white mica, which is clustered thickly along the ripple- 
marks. Although the prevalent type of rock is well-laminated, some 
of the coarser parts appear to be structureless. Drift- and current- 
bedding is fairly common. The constituent grains are angular and 
of two sizes, respectively about ;3>5 and 345 inch in diameter. 
2. Bala Sandstones.—These occur normally as beds averaging about 
2 feet in thickness, with interstratified shales and shaly sandstones. 
The thicker beds are nearer 3 feet across, but in some places, such 
as Nant Engyll Quarry, Coed Garth Eryr, Llwyn Bryn Dinas, and 
Llangedwyn Hall, lenticular masses, like those in the Llandilo, 
occur up to 10 feet in thickness. 
¢ i Ra in, AG 
Fic. 1.—Ball and pillow-form structures in beds of sandstone between 
cleaved shales, in crag west of Moelfre, Llansilin. 
When fresh the rocks are tough, blue, or blue-grey, fine-grained 
laminated micaceous sandstones, with many included lamine of shale. 
They are fossiliferous and occasionally slightly calcareous, when they 
weather to rottenstone. Frequently they are ripple-marked and 
show drift- and current-bedding, the latter structure being most 
common in the more lenticular masses. They are less felspathic than 
the Llandilo sandstones, but contain some thin layers of ash and 
isolated crystals of felspar. 
Pillow-form and Ball Structures. 
In some sections, either natural or quarried, parts of the sand- 
stone, interbedded either in similar sandstone or in shale, assume 
