THE ROCKS OF PLYMOUTH, I17 
shales, though examples of slaty cleavage occur, as at Deadman’s 
Bay, in the limestone itself, and in many of the ash beds. Some 
of these variations may be traced to locally-acting causes, others 
seem to have a wider origin. 
Sandstones.—Our sandstones and grits lie mainly to the south- 
ward of the limestone band (treated under the next head), and 
may be followed, more or less consecutively, for several miles in an 
east and west direction. Less varied than the slates, they still 
have a wide range of character. A very fine-grained red sandstone 
is associated with the limestone, and was found in considerable 
quantity at the time of the excavation for the Victualling Office. 
The limestone bed locally known as ‘“hard-head,” and chiefly 
used for road metalling, contains a notable proportion of quartz 
sand, 
Our most distinctive variety is the fine-grained, hard, micaceous 
sandstone of Staddon, chiefly dark-red, but in part grey. Passage 
rocks into this from the adjacent slates may be observed, in which 
sandy and clayey lamine alternate. The fact is important, as con- 
troverting the suggestion that these sandstones are unconformable 
to the rocks immediately around them. Sandstone beds of this 
series occur at Mount Edgcumbe, and continue on a western or 
north-western strike, the succession being kept up by coarser- 
grained grits, chiefly lilac, brown, or tawny. To the eastward the 
red hue is somewhat more persistent, but the general character of 
the succession remains much the same. Some of these grit beds 
are very compact. ‘There is an exposure at the ferry near Newton 
Ferrers of a reddish-grey rock, which may fairly be called a 
quartzite. It shows small flecks of mica on the joint faces, and 
under the microscope is seen to consist almost wholly of closely 
compacted quartz grains, with a little interstitial matter containing 
iron. 
These sandstones and grits are presumably all of Devonian age, 
with the possible exception of the fine red sandstone which fills 
some of the joints and cavities of the limestone—probably here, 
as at Torbay, a Triassic remnant. Be that, however, as it may, we 
have a distinct fragmental Triassic rock in a patch of conglomerate 
on the beach at Cawsand, in close association with the Triassic 
trap of that locality. There is evidence also of the extension of 
the Trias over a wider area in this neighbourhood, in a large block 
