THE GEOLOGY OF PLYMOUTH. 
455 
St. Budeaux of a narrow anticlinal— K 10° W. ; S. 10° E., with 
a cleavage nearly parallel to the northern dip, and striking H". 80° 
E. Between Egg Buekland and the Fort on the north, a northerly 
dip occurs N. 30° E., 60°; whilst, a short distance to the south- 
ward, near the church, we find it S. 10° W., 35°. There is 
another anticlinal at Blackpool, near Lynham ; and northerly 
dips are likewise found near Wolsdon and elsewhere. 
The rocks of our northern or lower group are chiefly slates, of a 
bluish gray or drab. Good roofing slate occurs at Cann Quarry, 
and a few other points. Near Plymouth there is an abundance of 
purple and greenish slate, largely variegated. By Saltash there are 
a few calcareous seams, one of which was worked many years ago, 
at Moditonham, for lime. Associated with the slate are numerous 
bands of trap rocks — greenstones and ash beds, some of which 
graduate into the slates by such fine degrees, that it is hardly 
possible to distinguish where the one ends and the other begins. 
The more important trap bands are near Saltash. Others, less 
pronounced, may be seen between Knackersknowle and Plymouth. 
There are still more traps at Compton, S willy, Eord, and Key ham ; 
and immediately to the north of the limestone — forming the hill on 
which the Devonport Column is built, where it rises at the junction 
of the limestone and the slate ; appearing in an almost identical 
position at the N.W. angle of St. Andrew's churchyard ; and 
stretching along the top of the hill from Bidgway by Chaddlewood. 
This trap-rock, called locally, in conjunction with some of the 
altered slate, dunstone, occurs in various forms. Near the surface 
it is generally of a dun or a reddish-brown, often vesicular, and 
commonly ferruginous, rotten through weathering. In depth it 
becomes bluish-gray in colour, and very hard. Occasionally it is 
amygdaloidal, as at Eord and Chaddlewood, where it contains large 
quantities of carbonate of lime, and may perhaps fairly be termed 
diabase. The sounder varieties are frequently used for building, 
and the tougher make excellent road metal. There are few hills 
in the area under review that do not contain either a trappean 
nucleus, or are not largely composed of one form of diorite or 
another. To its superior hardness, as compared with the slates in 
resisting denudation, no doubt many of these elevations owe some- 
thing of their present contour. 
Great part of these greenstones are undoubtedly the contempo- 
raries of the rocks wherewith they are associated, and therefore of 
