Percy F. Kendall—Tachylyte in Mull. 55d 
mittent in its outbursts, and cooling under widely different condi- 
tions, affecting the attraction or repulsion of its mineral constituents. 
The whole of these Lizard rocks bear the impress of this later 
mechanical and chemical change, but the broader features due to 
these original and subsequent causes should, I think, be distinctly 
separated and clearly recognized. 
VII. —Prenimmnary Novrres on some OcourRENCES OF TACHYLYTE 
In Mutt. 
By Percy F. Kenpatt, 
Assistant Lecturer on Geology at the Owens College. 
Al Nae following notes have been compiled in order that, pending 
the publication of the results of microscopic and chemical 
examination of the specimens collected, geologists visiting the 
island may have an accurate localization of the dykes and intrusive 
sheets bearing glassy selvages observed by the writer and Mr. J. 
Lomas, A.N. S S., during a mocene visit to Mull. The list includes, 
for the sake of completeness, three occurrences of tachylyte which 
have already been described, viz. :—Sorne and Gribun? and Ardtun.? 
The writer has not visited Sorne, but found the tachylyte at 
Ardtun, and re-discovered that at Gribun, in May, 1887. The last 
two exhibited certain peculiarities which appear to have hitherto 
escaped notice, and will be alluded to in the sequel. It is probable 
that some of the examples will prove upon examination to be more 
acid than the true tachylytes. 
In the table which follows, the word “country” is employed as 
a convenient term to indicate the rock through which a dyke or 
intrusive sheet passes, or, briefly, in the miner’s sense. 
Many dykes which once possessed glassy selvages have undergone 
a process of decomposition, which, while removing all trace of actual 
glass, yet, leaves characteristic appearances which are quite unmis- 
takable, though difficult to describe. Among the 200 or 300 dykes 
examined in Mull, a very large proportion are in this condition. 
The tachylyte is occasionally very local in its distribution, and 
may be present only in small, widely-separated patches. 
Asa general rule it is thickest on the outsides of the curves of 
sinuous ‘dykes (e.g. A. 12). 
The character of the “country” appears to have influenced the 
formation of glassy selvages, hard and compact rock being a better 
conductor of heat than loose decomposed or vesicular material has 
favoured their production. This has been observed in numerous 
instances of which A. 12 may be taken as the most conclusive. The 
dyke can be traced across the junction of a sheet of compact basalt 
with an underlying highly vesicular rock, and thence on into the 
compact rock again, and it can be seen that in the compact rock the 
tachylyte is $ in. thick, while in the softer rock it is reduced to a 
mere film. Occasionally, in following up a dyke the tachylyte 
1 Judd and Cole, Q.J.G.8., vol. xxxix. 2 Cole, Q.J.G.S., vol. xliv. 
