420 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
index to the coiTespondiiig variations in eoinposition and microscopic 
structure enumerated in the foregoing tabular arrangement. 
As a ride, the basic rocks which occur intrusively in connection with 
tire puys, esjiecially where they form a considerable mass, have as.surned a 
much more coarsely crystalline texture than those of similar composition 
which have been poured out at the surface. They are generally dolerites 
rather tlian basalts. But with this obvious distinction, the two groups have 
so much in common, that the geologist who passes from the study of the 
subterranean phenomena of the Plateaux to that of the corresponding phe- 
nomena of the Puys is at once impressed with the close relationship between 
the material which, in the case of the puys, has consolidated above ground, 
and that which has been injected below. There is no such contrast between 
them, for example, as that between the basic and intermediate lavas of the 
plateaux and the more acid intrusions associated with them. 
By fill- the largest number of the basic sills, bosses and dykes associated 
with the puys are somewhat coarsely crystalline dolerites without oli^•ine. 
They include, however, olivine-dolerites and basalts, and even some extremely 
basic compounds, (df these last, a typical example is supplied by the now 
well-known picrite of luchcolm, in the Birth of Borth, which occurs as an 
intrusive sheet among the Lower Carboniferous Sandstones.^ In recent 
years one or two other picrite-sills have been observed in the same district. 
An interesting example has been desciibed from a railway cutting between 
Iwlinburgh and Craniond where the rock invades and alters shales. More 
detailed reference to it will be made in the account of the sills connected 
with the puys. Another instance of the occurrence of this rock is in a 
railway cutting immediately to the west of Burntisland where it has been 
intruded among the Calciferous Sandstones below the Burdiehouse Limestone. 
Bocks approaching limburgite occur among the sills and bosses which 
pierce the Carboniferous Limestone series of Bile between Cowdenbeath and 
Inverkeithing. One of these is found at Pitaudrew, near Bordel Castle. 
Dr. Hatch observed that it consists of “ numerous porphyritic crystals of 
olivine, with a few grains of augite and an occasional small lath-shaped 
crystal of felspar imbedded in a groundmass which is composed principally 
of idiomorphic augite microlites, small crystals of a brown mica, granules of 
magnetite and prisms of apatite. In addition, there is a considerable 
amount of interstitial matter, which is partly colourless glass, and partly 
shows a slight reaction between crossed nicols.” Another example of the 
same type of rock occurs as a plug or boss in the tuff-vent of the Hill 
of Beath, and a further display of the limlrargite type is to be seen in 
Dunearn Hill near Burntisland. 
Although olivine-basalts of the Dalineny type are most frecpiently met 
with as interstratified lavas, they also occur as bosses and silks. The typical 
example from Dalmeny is itself intrusive. Other illustrations are to be 
found in the Castle Eock of Bdinlmi-gh and in the sheets near Crossgates 
and Blairadam in Bile. The presence or absence of olivine, however, may 
^ IVana. Itoij. Soc. Bdin. vol. xxix. (1879} p. 506. Teall, Britisk Petrograiihy, p, 94, 
