240 J.J. H. TEALL—PETROLOGICAL NOTES ON 
Two dykes, presenting a type of structure in some respects dif- 
ferent from that of any of those previously described, are exposed 
on the east side of the Tarret, near High Green, a farmhouse about 
5 miles N.N.W. of Bellingham. My attention was first called to 
these dykes by Prof. Lebour, and subsequently Mr. Howell was kind 
enough to mark them in for me on my maps. 
The first to be described strikes W. 10° S., E. 10° N., and crosses 
the Tarret Burn at a point W.S.W. of High Green. It is well ex- 
posed in the bed and along the sides of the stream, where it is 
about 50 feet thick. The central portions are coarsely and uniformly 
crystalline in texture; but the margins, where not highly altered, 
are finely crystalline or even compact. The only elements which 
can be distinctly made out with a hand-lens are felspar and pyrites, 
the latter mineral occurring very sparingly and only in certain 
specimens. 
Its specific gravity lies between 2°9 and 2°93, and its chemical . 
composition, as determined by Mr. Stead, is as follows :— 
Silica: oni tee Nene ede Nets ee ems 53°70 
AT TEE ee yas ee tee ce ewe 16221 
Perme Ode: See crs 10°64 
Time. ea Cee ee 6:66 
Mae nesia fos te an tente eta 5°24 
Potashs "ieee a ee eee 0-99 
SOGa a ysid it, so eteteare teens 2°65 
Water cee. ce ee eee 55 
99-64 
One of the most interesting features connected with this dyke is 
its mode of alteration at the north margin. The rock there shows 
a cellular structure, evidently connected with its mode of jointing. 
The walls of the cells are formed of a deep brown and comparatively 
hard substance, evidently rich in hydrated ferric oxide, while the 
cells themselves are filled with a friable white or cream-coloured 
material. On analysis these two substances yielded to Mr. Stead 
the following results :— / 
Cell-walls. Cell-contents. 
Silica * "25 papas eee 43°50 66°20 
Alumina eee ee 14°61 23°92 
Ferric oxide...... aylel iat 0-78 
Tate ye ke ee 0-44 0:95 
Maenesia.. ) a. 0-11 0°33 
Carbonic acid .... Laie traces 
EGbaighl ts oe certs 0:09 0-04 
Soda. es cs heer 0-26 0:57 
Water "..;. Seca ee 9°50 7:00 
99-65 99°79 
On comparing these analyses with each other, and with the 
analysis of the comparatively unaltered rock, we see that the effect 
