350 G.W. Tyrrell— Tertiary Dykes 



the Pygasteridae, so far from converging across the ambulacrum, 

 actually diverge, while Diademoid processes, even when they do not 

 meet in an auricular arch, are always inclined towards one another. 



These three points of difference are all in a sense prophetic of the 

 subsequent perignathic changes in Holectypoida and Clypeastroida. 

 The wide basal separation and increasing divergence of the processes 

 suggest the future shifting of these structures to an interradial 

 convergence as seen in Clypeaster, and more completely in 

 Echinocyamus or Echinaraclinius. The situation of the processes of 

 Plesiechinus well back from the peristomial margin is found in 

 a greater degree in the Clypeastroida, and implies a considerable 

 inclination of the lantern away from its typically vertical position in 

 llegular Echinoidea. 



The interradial ridges, feeble though their development is, have 

 strong points of resemblance with the homologous structures of 

 Discoides and Conidas, as I hope to show in a forthcoming paper. 



It thus appears that the perignathic girdle of the Pygasteridae 

 shows a very close correspondence with that of phylogenetically or 

 ontogenetically young Diademoida as regards the essential disposition 

 of its elements ; but that in its details it already indicates 

 a progressive tendency towards the Clypeastroid type of girdle. The 

 development of the short interradial buttresses, though clearly an 

 adaptation to the requirements of the thin invaginated peristome, 

 represents another foreshadowing of a typically Clypeastroid feature. 



III. — Some Tertiart Dtkes of the Clyde Area. 



By G. W. Tyrrell, A.E.C.Sc, F.G.S., Lecturer in Mineralogy and 

 Petrology, Glasgow University. 



(Concluded from the July Number, p. 315.) 

 Leidleitk and Inninmorite. — Typical rocks of these groups are 

 as yet unknown in the Clyde area. A N.JN". W. dyke at Barrassie 

 Sands, near Troon (Ayrshire), has points in common with leidleite. 

 It shows anorthite phenocrysts in a subvariolitic groundmass of 

 acicular felspar and augite, enveloped in a partially devitrified glassy 

 base. A north-south dyke at the north end of Loch Fad, Bute, may 

 perhaps be regarded as a doleritic end-variety of leidleite (see 

 Anderson & Radley, 1916, fig. 2b, p. 208). A pitchstone-like dyke 

 from the Smurig Burn in the south of Arran has all the characters 

 of leidleite, except that it carries rather abundant granular augite, 

 instead of acicular crystals. 



Tholeiite (Hrunton Type). — This type has been based by the 

 petrographers of the Geological Survey on the so-called " augite- 

 andesite " of the Brunton dyke, Bingfield, Northumberland, figured 

 by Harker (1908, fig. 54, p. 203), and described and figured by 

 Teall (1884, pp. 236-7). It is abundant among the Tertiary dykes 

 of Mull, and two analyses of the type have recently been recorded 

 (Geological Survey, 1914, p. 82; 1915, p. 55). In the brief notes 

 accompanying the analyses these rocks are said to be composed 

 respectively of " small clots of augite and acicular crystals of 

 plagioclase, with interstitial dark-brown glass", and of "augite, 



