THE GEOLOGY OF PLYMOUTH. 
457 
so easy to trace these repetitions as one would desire. Still, there 
is evidence. The parallelism of the bands of trap near Saltash 
may be due to this cause. But that to which I would direct 
especial attention is the grouping of the purple and variegated 
slates in and near Plymouth. You will find them in the railway 
cutting at the Friary, at Lipson, in the railway cutting at Eosehill, 
and again beyond the little tunnel to the west of the Stoke Station. 
Each group of coloured slates is separated from the next by slates 
of the ordinary character; and I believe, that instead of having 
four sets of coloured beds, we have one set four times repeated ; so 
that the total thickness of the strata exposed, instead of being 
2,000 yards, is somewhere about 300. Moreover, this hypothesis, 
if correct, applies more or less to the whole of the rocks of the 
division. 
Before passing on to the limestones, the occurrence of elvans 
should be noted. Elvans are rocks of granitic character, which 
fill fissures, and form dykes running across the country. They are 
thus intrusive. One elvan may be seen at Cann Quarry. There 
is another, porphyritic, at Roborough Down, which attained con- 
siderable local notoriety for building purposes under the name of 
Roborough Down stone. A third occurs near Jump. They cut 
through the slate beds, and have a general direction east and west. 
Except in the vicinity of Saltash, there are no fossils locally in 
the rocks of our lower group, which in part are really Lower 
Devonian. Elsewhere important discoveries have been made ; and 
the Lower Devonian rocks of Looe, Polperro, and Fowey, have 
yielded fish remains identical in species with some which occur 
characteristically in the undoubted Old Red. The sea bottom here 
may have been of greater depth. 
We now come to the middle Plymouth group. "When, many 
years ago, it was first stated that the Plymouth limestone contained 
fossils, the idea was scouted as absurd, though thousands are re- 
vealed in the pavements over which we daily tread, and though in 
the limestone cliffs which bound our shores corals eroded into 
prominence by the action of the sea and spray constantly occur. 
It required, however, long-continued labour on the part of Mr. 
Hennah fully to establish this fact. Now we know that our lime- 
stone is in great part made up of organic remains. It is in fact an 
ancient coral reef. A physical analysis of a fragment of Plymouth 
limestone by Mr. Sorby gave the following result : 
