ON THE CRYSTALLINE ROCKS OP THE LIZARD DISTRICT. 477 



(1) In some cases the dioritic rock is pierced by veins of the 

 granitic, which may be of any thickness from a few inches to a few 

 lines ; occasionally the former is completely brccciated and the 

 pieces are separated by the latter rock, the intervals also varying 

 in thickness in like way. Thus in the Granulitic Grouj) we find 

 sections which closely resemble those where an igneous rock breaks 

 up and includes another igneous* or a massive sedimentary rock. 



(2) In other cases the two varieties, for considerable distances, 

 appear perfectly interstratified, and exhibit regular bands of the one 

 or the other which vary in thickness from several inches to a small 

 fraction of an inch, with occasional layers of a rather intermediate 

 character f. In the latter case the lines of junction, though fairly 

 sharp, do not resemble ordinary intrusive junctions — there are no 

 indications that the one rock has been broken by the other. The 

 structures of the two are similar, and the one seems to pass into the 

 other by a very rajjid mineral change. 



(3) The thin slices under the microscope do not exhibit either 

 that mixture of larger and smaller grains, or the peculiar minute 

 ** mosaic " structure, which commonly occur when a rock already 

 crystalline has been crushed. The structure is not that charac- 

 teristic either of the " newer gneiss " series of Glen Logan, or of 

 one of the crushed granitoid rocks common in the Central Alps ; 

 though to these it occasionally presents a very faint resemblance. 

 Xor is it that of the Saxon granulites. The present structure, 

 whether original or secondary, seems to have been assumed in situ. 



(4) Between the two extremes mentioned in (1) and (2), every 

 intermediate form can be discovered. The angular dioritic frag- 

 ments appear to be gradually flattened or elongated till they become 

 lenticular streaks or even bands, and the vein-like intercalations 

 of granite appear to be drawn out with them into similar bands, 

 very much as a mixture of glass of two colours can be drawn out 

 when it is heated until it becomes viscous. 



' These conditions appear to be best fulfilled by the following hypo- 

 thesis : — that into a basic magma, which at any rate was sufiiciently 

 solid to break into fragments, an acid magma, at a very high tempe- 

 rature, was injected, — that either the more basic material was still 

 somewhat plastic when this intrusion took place, or it was, by this 

 -accession of heated stuff j. so far softened that it was drawn out 

 into streaks, and was even sometimes slightly mixed with the other 

 by actual fusion, when movements occurred in the mass ; and 



* This, so far as my experience goes, is rather rare and local in its occurrence. 

 The most remarkable instance which I have seen of the brecciation of one 

 igneous rock hy another was at the Corporation quarries, Montreal, where 

 the nepheline syenite is shattered by and embedded in a rather compact dark 

 rock, perhaps a tephrite. — T. Gr. B. 



I have seen some striking illustrations of the complete brecciatibn of gneiss 

 by granite in Spiti.— C. A. J\PM. 



t This structure is rather more conspicuous in Pen Voose Cove ; the former 

 in Kennack Cove. 



I Probably tbe temperature of solidification in the basic rock would b9 

 considerablv lower than that of the acid rock. 



