560 Dr. C. Callaway—On Blake’s “ Monian System.” 
gives place to a black or greenish substance having a hardness of 
about 2. 
Several cases have been noted in which small strings have been 
given off consisting wholly of tachylyte, for example strings 
descend from the Ardtun intrusive sheet between the colums of the 
basalt ‘country ”’ to a distance of, at the least, four feet. 
A good test for intrusive sheets when the stratigraphical evidence 
is inconclusive has long been a desideratum, and it may be that 
certain peculiarities in those observed in Mull may prove really 
diagnostic when a sufficient number of examples have been examined 
to justify generalisation. 
The Ardmor intrusive sheet (S. 1) was recognized as such in 
the first instance by the extraordinary sharpness of the plate-like 
columns and subsequently the determination was confirmed by the 
discovery of the tachylyte selvages. This sheet is amygdaloidal 
in the middle and the same peculiarity is presented by other intrusive 
sheets, and also by many dykes (eg. G. A.) in Mull, while, as is 
well known, lava-streams are vesicular top and bottom. 
In conclusion, the writer desires to express his conviction that, 
when Mull comes to be fully surveyed, dykes with tachylyte 
selvages will be numbered by hundreds, or, it may be, even by 
thousands. 
Note by Bernard Hobson, Esq., B.Sc.—On the coast, between tide- 
marks, to East of the “principal ravine” of Mr. Starkie Gardner, at 
Ard Tun, are two intrusive sheets, each about one foot thick, one 
above the other, with only 2 or 3 inches between them. They are 
intruded into the basalt, and each sheet bears a thick selvage of 
“tachylyte both above and below, the mass of the sheet being charac- 
terized by large spherulites, clearly visible to the naked eye, 
especially when the rock is wet. The sheets are irregularly colum- 
nar and their surfaces where exposed by erosion of the “country,” 
present a striking appearance owing to their brilliant glassy black- 
ness. 
VIII.—Nores on THE “ Montan System” or Proressor BLake. 
By Cu. Cattaway, D.S8c., F.G.S. 
S Professor Blake, in his elaborate paper in the Quarterly Journal 
of the Geological Society for August last, makes frequent 
reference to my work in Anglesey, I may perhaps be permitted a 
brief comment. I do not intend to enter upon an elaborate con- 
troversy, since I prefer, now that the two views of the district have 
been published, that those interested in the matter should visit the 
ground, and judge for themselves. Nevertheless, it is well that the 
salient points of agréement and difference between the two readings 
should be placed in a clear light. 
On the fundamental question—the Archean age of the bulk of 
the Anglesey crystallines and altered slates—Prof. Blake is at one 
with Dr. Hicks, Prof. Bonney, and myself; but he differs from those 
geologists who place a part of our Archean in the Ordovician, and 
