IX THE SYENITE OF BR.YDGATE PARK. 105 



crystal, may bo for a considerable distance i)ractically a single 

 crystal. This, I take it, is the case where a spherulitic or " graphic '' 

 structure is setup. These last structures, then, seem to indicate that 

 the temperature of the mass was comparatively low, but that it fell 

 very slowly. Its constituents, so to say, have ample time to arrange 

 themselves, but the magma is not sufficiently fluid to permit of the 

 molecules travelling for any distance. We may illustrate it by a 

 tightly-packed crowd, where individuals already almost touching 

 one another may contrive to get together into little knots, but when 

 this is done these knots are effectually isolated one from the other. 

 Further, the larger crystals of felspar which already exist form, as 

 it were, rallying-points for the felspathic constituents in the neigh- 

 bouring glass ; or, to speak more exactly, the formation of crystals — 

 as is well known — is promoted by the presence of solids, especially 

 when these are of like composition. So we find that most (perhaps 

 all) rocks which exhibit a micrographic * structure are also porphy- 

 ritic. In this case we frequently observe that the porphyritic 

 felspars are no longer, strictly speaking, idiomorphic ; for their 

 crystals pass abruptly into a " graphic " growth of felspar, separated 

 by quartz. Sometimes, indeed, we observe that the original, com- 

 monly idiomorphic, felspar can still be detected by some difference 

 in its optical characters or amount of decomposition. About this 

 has been formed a zone of felspar, apparently not always quite 

 identical in composition, and this zone throws out root-like prolon- 

 gations, which are sometimes, but by no means always, in optical 

 continuity with it. Probably this diversity of habit is mainly 

 determined by accident, which favours the starting of this growth 

 at particular points. The wedge-like outline frequently assumed by 

 the felspar in rocks with a micrographic structure is probably due to 

 the attempt on the part of the mineral to assume an idiomorphic 

 form. The less regular shapes, such as the branching or root-like 

 varieties, may be due either to the resistance offered by the almost 

 solid (and crystallizing silica), or in certain cases to some slight move- 

 ments of the mass in the last stages of solidification t. In relation 

 to this question we may notice that at a junction-surface between a 

 holocrystalline rock and a sedimentary one there is not seldom a zone, 

 perhaps about Jq- of an inch wide, in which the felspars seem to 

 spring from the junction-surface and grow inwards, like tufts of 

 grass from the ground. 



To come, then, to the case of these *'s3'enites" of Charnwood. 

 As already noticed by myself X, and as described in more detail by 

 Mr. Teall §, they are characterized, especially in the case of the 

 apparently coarselj^-crystalline masses of Groby, Bradgate, Markfield, 



* ' Micropcgniatitic ' of some authors. 



t I must not, however, be understood as pledging myself to the assertion that 

 this structure can only be produced in a cooling mass. There are cases where 

 there is much to be said in favour of its secondary origin. But this is no more 

 than may be affirmed (for instance) of spherulitic structure. (See my Presiden- 

 tial Address, Quart. Journ. Geol. Soc. 1885, Proc. pp. (58, 09). 



I Hill and Bonney, Quart. Journ. Greol. Soc. vol. xxxiv. (1878) p. 215. 



§ ' British Peti-ography,' p. 270. 



