72 PROP. J. W. JTJDD ON- TERTIAEY 



enstatite, augite, &c., which are found in some basalts as having 

 separated at some period from the magma, out of which the enclosing 

 basalt was itself formed, may regard these as examples of the 

 glomero-porphyritic structure on a gigantic scale. 



When the character of porphyritic crystals is compared with that 

 of the same mineral forming a part of the ground-mass of the rock, 

 we usually find that the porphyritic crystals are of more basic 

 composition. Thus, in a dolerite containing labradorite-crystals in 

 its base, the porphyritic crystals appear to be anorthite ; this is 

 proved both by the extinction-angles of the different felspar- crystals, 

 and by the application to them of Szabo's chemical test. The same 

 fact has been well illustrated in the case of the dolerites of Iceland by 

 M. Br eon *. In the same way, basalts which contain porphyritic 

 crystals of labradorite sometimes appear to have a more alkaline 

 species of felspar (andesine or oligoclase) in their base. 



It has frequently been remarked that the enclosures of glass in 

 porphyritic crystals have a different character from the glass of the 

 ground-mass of the rock. But while we find this to be the case in 

 some of the rocks we are now considering, we also find evidence that 

 the material of the ground-mass has in many cases penetrated into 

 cracks and eaten into the surfaces of the porphyritic crystals. There 

 appears to be evidence that at the time when this took place, the 

 materials of the ground-mass had already in part crystallized. Thus 

 we sometimes find crystals of magnetite or granules of augite which 

 seem to have been mechanically forced between the plates of a por- 

 phyritic crystal of felspar. 



5. Micro-pef/matitic (?) Structure. — There is yet another structure 

 not unfrequently displayed by these rocks which is worthy of very 

 careful study. In 1874, AUport drew the attention of geologists to 

 a very interesting structure, which he figured from the Rowley-ragf. 

 The pyroxene and felspar are here seen intergrown in such a manner 

 that the former constitutes a number of rays diverging from a centre 

 in the midst of the felspar-crystals. I have found beautiful examples 

 of this structure in a coarse dolerite from near Bombay, for speci- 

 mens of which I am indebted to Mr. A. P. Young, F.G.S., but have 

 not noticed it in any of the rocks now under consideration. Fox a 

 similar intergrowth of minerals, H. Pischer proposed the name of 

 " ocellar structure," while in 1881, F. Becke described a number of 

 remarkably interesting cases of the same character under the name 

 of centric structure. 



Among the rocks exhibiting what Michel-Levy calls micro- 

 pegmatitic structure, and Eosenbusch granophyric structure, there are 

 some which exhibit a plumose and radiating arrangement of the 

 two intergrown minerals. M. E. Breon has figured a beautiful 

 example of this kind from Iceland under the name of microgranulitic 



* Notes pour servir a I'etude de la Greologie de I'lslande et des lies Foeroe 

 (1884), p. 18, pi. ii. fig. 2. 



t Quart. Journ. Geol. Soc. vol. xss. (1874) p. 549, pi. xxxiv. %. 28. 



