141 



been especially recorded by ZIRKEL (1) and JUDD (2) as occurring in the gabbros 

 of the WBst of Scotland. They occur in fissures or bands parallel to fissures, 

 and are often connected by minute ramifying tubular processes. The same 

 felspars often contain the minute black rods and plates which have been 

 already referred to (page 28) as occurring along definite planes within 

 the crystal. 



In some of the rocks of the basic group which exhibit a well marked 

 trachytic texture the porphyritic elements in the rock are not simple crystals, 

 or fragments of such crystals, but granular aggregates to the external 

 surfaces of which felspar-substance has been added so as to produce definite 

 external form. This is well seen in the porphyritic portion of the Tynemouth 

 Dyke.' 8 > 



In this case granular aggregates of a felspar allied to anorthite have 

 been completed externally by felspar- substance having optical characters 

 somewhat different from those of the principal mass. This substance has 

 doubtless been added during the later stages of consolidation, and is possibly 

 contemporaneous with the felspars of the groundmass. The individual grains 

 of what may be termed the primary aggregate are related to each other 

 in exactly the same way as the grains in many typical gabbros. It seems 

 reasonable therefore to suppose that the aggregates were formed under 

 plutonic conditions ; that they were then broken up and carried forward 

 by movements taking place after consolidation had progressed to a certain 

 extent ; and that the external zone, which determined the crystalline 

 faces, was added during one of the latter stages in the process of con- 

 solidation. 



Professor JUDD describes and figures under the name of glomero- 

 porphyritic structure a very interesting feature which occurs in the dolerite 

 of Fair Head, Co. Antrim. In this case granular aggregates of anorthite 

 and olivine occur as porphyritic elements in a groundmass which may be 

 described as ,an ophitic olivine-dolerite. (4) 



Felspar in the form of microlites and skeleton crystals occurs in 

 those rocks which contain a considerable amount of glass or other interstitial 

 matter. The precise determination of the felspars of the coarse-grained rocks 

 and of the porphyritic felspars of the fine-grained rocks is best effected either 

 by the observation of the optical characters presented by cleavage flakes ; by 

 SZABO'S method depending on the simultaneous observation of flame 

 colouration and fusibility ; by the determination of the specific gravity ; or 

 by micro-chemical tests. It can only be made with certainty in ordinary thin 

 slices by the recognition of a section whose direction can be definitely 

 determined by a study of the cleavage cracks, external form, or the relation 



(1) Geologische Skizzen Von der Westkuste Schottlands. Z.D.G.G., Vol. XXIIL, (1871), 

 p. 59. 



(2) On the Tertiary and Older Peridotites of Scotland. Q.J.G.S., Vol. XLII.., (1885), 

 p. 375, 



(3) TEALL. North of England Dykes, Q.J.G.S., 1884, p. 247 and Plate XIII., Fig. 1. 



(4) Q.J.G.S., 1886, Plate VII., Fig. 3. 



