246 JOURNAL OF THE PLYMOUTH INSTITUTION. 
outline, though they do not manifest any approach to crystallization. 
Much the same talcose appearance as the body of this rock 
displays is seen in the killas of Hingston Down when subjected 
to great heat for the purpose of vitrification, while that process is 
incomplete. Specks of mica occur in the rock so treated, and 
there are observable what may be the beginnings of nodular 
accretions, while when the process is finished it is attended by the 
distinct if imperfect development of quartz and felspar crystals. 
At Meavy too occur spotted slates, with a red cast, containing 
an appreciable amount of iron (there are thick ferruginous red 
slates or flags on the granite border also at Lee Moor), and in 
another phase of alteration the rock is patched with irregular 
nodular accretions, and strings of micaceous matter, a middle stage 
apparently between the andalusite-slate and the well-defined mica- 
schist. 
Besides the “ribbon-jasper” of Ivybridge and Peek Hill, 
already noted as an altered rock, there should be recorded the 
existence within our area of ‘ porcelain-jasper.” This is not true 
jasper, but baked and highly-indurated clay-slate, and is found 
at the junction of some of the intrusive igneous rocks. It is 
plentiful in the vicinity of Lydford. It derives its name from 
its usual fine-grained, close-textured, porcellaneous look. The 
Lydford variety is, as a rule, pale-green or bluish, and commonly 
banded, but different in character from the “ribbon-jasper.” 
I have found a red porcelain-jasper also on the flank of the 
eruptive boss at Yealmpton, with some portions of the original 
clay-slate attached, thoroughly melted and scorified. 
SUMMARY. 
The number of distinct species of rocks occurring in the 
Plymouth district, as defined in this paper and enumerated here, 
is upwards of forty, while the varieties would make a very 
much larger total. From this it will be seen how well founded is 
the claim put forward for this locality, as possessing an exceptional 
petrological interest. 
The number of minerals, distinctly identified as entering into 
the constitution of these rocks, several of which have only been 
recognised by the microscope, approaches fifty, including: 
achroite, actinolite, asbestos, andalusite, apatite, arragonite, augite, 
