CHAP. XXXIV 
PETROGRAPHY OF THE DYKES 
12 7 
usually somewhat abrupt, insomuch that on weathered faces it is often 
difficult to get good specimens, owing to the tendency of the vitreous 
portion to fly off when struck with the hammer. The glass doubtless 
represents the original condition of the rock of the dyke. It was 
suddenly chilled and solidified by contact with the cold walls of the 
fissure. Inside this external glassy coating, the molten material could prob- 
ably still move, and had time to assume a more or less completely crystalline 
condition before solidification. Hot infrequently the glass shows spherulitie 
forms, visible to the naked eye, and likewise a more or less distinctly 
developed perlitic structure. These features, however, are best studied in 
thin sections of the rock with the aid of the microscope, as will be sub- 
sequently referred to. 
In some dykes, the glass is not confined to the edges, but runs in 
strings or broader bands along the central portions, or has been squeezed into 
little cavities like steam-holes or into minute fissures. One of the most 
remarkable examples of this peculiarity occurs in the well-known dyke of 
Eskdale, which runs for so many miles across the southern uplands of 
•Scotland. 1 This dyke throughout most of its course is a crystalline rock of 
the andesitic type. At Wat Carrick, in Eskdale, it- presents an arrangement 
into three parallel bands. On either side, a zone about eight feet broad 
consists of the usual crystalline material. Between these two marginal 
portions lies an intercalated mass 16 to 18 feet broad, of a very compact’ and 
more or less vitreous rock. The demarcation between this central band and 
tbe more crystalline zones of the outside is quite sharp, and the two kinds 
'd rock show a totally distinct system of jointing. There can, therefore, be 
little doubt that the glassy centre belongs to a later uprise than the outer 
Portions, though possibly it may still have been included in the long pro- 
cess ol solidification of one original injected mass of molten material. If 
ne marginal parts adhered firmly to the walls, the centre, which with its 
and of vesicles seems often to have been a line of weakness, might be 
ruptured and subsequent intrusions would find their way along the rent. 
samples ot this splitting of dykes with the intrusion of later eruptive 
Material will be cited in later pages. 
Plough, while mapping for the Geological Survey the extra- 
Fi munerous dykes in the eastern part of Argyleshire between the 
( j , 1 Elyde and Upper Loch Fyne, observed six or seven examples of 
} ms showing glassy bands in their centres, with characters similar to 
• ° Se ° f kh e Eskdale dyke. He found an absence of definite and regular 
. muts in the central glassy band, and on the other hand, an irregular set of 
visional planes by which the rock is traversed, and which he compared to 
°se seen in true perlitic structure. 
grai^Wn ^ a general rule > the external portions of a dyke are closer- 
crvsthV 1 1M1 the CeiltrC ’ rare cases occur where the middle is the most finely 
as , lle P ai t- I am disposed to regard these cases and the glassy centres 
^ reality no true exceptions to the rule, that the outer portions 
1 See Proc. Roy. Phys. Soc. Edin. v. (1880), p. 241. 
