Q. W. Tyrrell — Alkaline Igneous Rocks, West Scotland. 127 



condition in the same interspace. A typical example of these rocks 

 occur in a Tent exposed in the River Irvine, near Hurlford. This 

 consists predominatingly of anhedral grains of purple augite with 

 a little magnetite in an apparently continuous colourless groundmass, 

 which, in polarized light, breaks up into laths of plagioclase, isotropic 

 analcite, and low-polarizing nepheline. Both the latter occur in 

 interstitial areas in which the nepheline occupies the margins, and 

 projects with good crystal form into more or less decomposed analcite 

 occupying the central space. The nepheline has the characters of 

 ' nepheline x ', described by Bailey, and is accompanied by a mineral 

 of much the same appearance but lower refractive index, just as in 

 the East Lothian occurrences.^ These rocks may be termed analcite- 

 nepheline-lasanite. With increasing abundance of nepheline and a 

 corresponding diminution of analcite and felspar, especially the latter, 

 these rocks approach closely to the true nepheline -basalts. A rock from 

 the Alton Burn, Tarbolton, is generally similar to the above, but is 

 much richer in nepheline. The latter forms small plates poikilitically 

 enclosing granules of augite. Small flakes of biotite occur abundantly 

 in the groundmass, as well as apatite in large stumpy crystals, 

 averaging one-tenth of an inch in diameter. 



The true monchiquite lavas are well represented by a beautifully 

 fresh rock from the burn near Stevenston, two-thirds of a mile north- 

 east of Ochiltree. It consists of a crowded mass of small euhedral 

 grains of titanaugite and sparse minute specks of magnetite embedded 

 in a scanty base of analcite. Olivine is very abundant in small 

 porphyritic crystals, partly fresh, partly hsematized. A few small 

 phenocrysts of augite also occur. The analcite tends to segregate into 

 small rounded areas which have tangentially aligned prisms of augite 

 around their margins. In the larger areas the mineral shows 

 anomalous double refraction similar to that of leucite, and the cubic 

 cleavage is well marked. Other slides from the same locality show 

 large ocelli of augite with a nucleus of analcite carrying detached 

 aggregates of augite grains and numerous minute crystals of segirine. 

 The groundmass of this rock is largely composed of fresh nepheline 

 which forms small plates poikilitically enclosing the other constituents. 

 We have, therefore, the rare type nepheline-monchiquite . 



Limhiirgite, characterized by a glassy base, is known from one 

 locality only, the hardened rim of the little vent at the Earthwork, 

 Barnweill, near Tarbolton. This mode of occurrence suggests that 

 the glass in the rock is due to the refusion of original analcite in a 

 monchiquite lava. The glass is distinguished from analcite by the 

 lack of cleavage, its dark-brown colour, and by the abundance of 

 microlites of magnetite. The presence of glass in place of analcite is 

 the only difference between this rock and the monchiquite described 

 above. 



Petkologt. 



It is not possible here fully to discuss the petrology of these rocks 

 considered as a homogeneous suite or province possessing certain 

 characters in common, and otherwise bearing indications of a common 



^ Geology of East Lothian, 1910, p. 110. 



