418 
HARKER : PETROLOGICAL NOTES. 
border and traversed by irregular cracks. They are sensibly 
pleochroic Some show a well-marked cleavage, and when this 
is seen, the crystals extinguish parallel to the cleavage-traces, and 
give the deepest absorption for vibrations in that direction. They 
may be due to the alteration of biotite, of which one or two fresh 
flakes are present, but whether they consist of allanite or some other 
uncommon mineral I am not able to state with confidence. 
No. 134 (not sliced) is a similar rock, though with a rather more 
generally crystalline appearance and with fewer porphyritic felspars. 
It has a general brown colour, and is penetrated by little red veins of 
ferric oxide, presenting altogether a close resemblance to some of 
the " porphyrites " of the Cheviots, which have certainly supplied 
numerous boulders to our east coast. It must be remembered, how- 
ever, that lavas of a not dissimilar type are extensively developed 
in the Old Red Sandstone and Carboniferous of southern Scotland, 
and such specimens as Nos. 174 and 202 cannot be confidently 
traced to their source until these rocks are very thoroughly known. 
I select next a few samples from the very numerous dolerites 
and diabases, which for the most part present few distinctive features 
in hand-specimens, wdiile even under the microscope they only 
occasionally show sufficiently marked characteristics to be safely 
identified with known rocks. No. 15 has a dull, dark-grey ground- 
mass, enclosing yellowish-white felspars up to a quarter or half an 
inch long, Avith evident twin-striation, and little black spots of a 
pyroxenic mineral. 
Micro. [1052]. The porphyritic felspars show Carlsbad, albite, 
and pericline twinning, and some degree of zonary banding. The 
pyroxene is found to be of a rhombic kind, a gTeen pleochroic hyper 
sthene, with a rather unusual mode of alteration. It is very abundant, 
and little octahedra of magnetite also occur in some quantity. The 
rest of the rock consists of little lath-shaped, striated felspars, with 
partial fluidal arrangement. 
Another specimen. No. 562, from near the Matron, shows to the 
eye very similar characters. Many of the felspars have a rounded 
outline, and this, with the other characters of the rocks, recalls the 
peculiar lavas of Eycott Hill, in Cumberland. The resemblance is 
