106 PROF. T. G. BONNE Y ON A CONTACT-STRUCTURE IN 



and HaminerclifF, by the prevalence of a micrographic structure ; that 

 is, they are not really a uniform holocrystalline rock, but a porphy- 

 ritic rock, with abundant crystals set in a micrographic matrix *. 

 We can thus assign a reason why this Bradgate syenite has so 

 little affected the neighbouring sedimentary rock, while at Brazil 

 Wood the slaty lock is very highly altered near the junction with 

 the granite, and appreciably so when it is last seen at a distance of 

 thirty yards. But the Mount-Sorrel granite is a holocrystalline 

 rock, and continues to be the same at the junction in Brazil Wood ; 

 that is, we may assume that either the temperature of the sedimen- 

 taries had been so much raised before the granite was intruded 

 that it cooled very gradually, or (what is perhaps only another way 

 of stating the same thing) that the granite was at a very high 

 temperature. At any rate, the fact that we do not find indications 

 of a selvage to the igneous mass indicates that it did not lose heat 

 rapidly. 



But in the Cbarnwood syenite, mentioned above, we find that 

 a micrographic structure is prevalent throughout the mass, and 

 this, near the junction, is replaced by an ordinary "trachytic" 

 structure or by one which seems to indicate the devitrification of a 

 glass t. Here also we notice that the embedded crystals are ofteD 

 broken, so that the rock almost presents a fragmental aspect. I infer, 

 then, that this mass was, as a whole, at a comparatively low tempera- 

 ture, and consisted, when it reached its present position, of solid 

 bodies (i. e. crystals), amounting to at least half the volume of the 

 mass. 



But, as has been said, some rocks are almost uniformly holo- 

 crystalline. In connection with this it is interesting to notice a 

 structure which might almost be called characteristic of vein-granites, 

 and is not seldom seen to form a kind of selvage, often not more than 

 •2" thick, to a fairly coarse holocrystalline mass at its junction 

 with a sedimentary rock (which is always much altered). In such 

 cases the crystals are rather small ; the felspar often is only partially 

 idiomorphic, and sometimes there seems to have been a " neck-and- 

 neck race " between it and the quartz, and the latter has occasionally 

 contrived to win. This produces a sort of mosaic of quartz and 

 felspar, resembling the microcrystalline structure in many felsites, 

 and even bearing some likeness to certain cases of contact-meta- 

 morphism in sedimentaries. The minerals sometimes almost dove- 



* Among British rocks which afford excellent examples of this structure may 

 be mentioned parts of the so-called ' Dimetiau ' of St. David's, and of the granitoid 

 rock of Ercal Hill (Wrekin). Formerly, in consequence of certain anomalies in 

 structure, I regarded these as not of igneous origin, but further study and 

 wider experience have convinced me that I was wrong. 



t A specimen collected from the little pit on Holgate Hill several years ago, 

 which was believed to be, if not actually in contact with the sedimentary, all 

 but touching, shows a curious mixture of ' trachytic,' subspherulitic, and imper- 

 fect micrographic structure. One lately collected, probably within a yard of 

 the actual junction, shows the micrographic structure, though it is on a very 

 small scale and less regular than in an average specimen of the syenite frou) 

 liradgate Park and elsewhere. 



