NEIGHBOURnOOD OF SARN, CAERNARVONSHIRE. 453 



blende. It appears to differ from the more ordinary " intergrowth," 

 in that the whole of the augite was formed before any part of the 

 hornblende. When both the augite-nucleus and the hornblende- 

 border are twinned, the twin-planes, though of course parallel, are 

 not necessarily coincident. Sometimes, as on the south-east side of 

 the crest of Mynydd-y-graig, the hornblende thus formed has idio- 

 morphic crystal-boundaries, and its original nature is conclusively 

 demonstrated. But when once admitted and looked for, this 

 complementary hornblende is found to be by no means uncommon : 

 it is distinguished from the perimorphic hornblende by its boundary 

 against the augite being much less irregular, and from original 

 hornblende enclosing augite-grains by the correspondence in crys- 

 tallographic orientation of the two minerals. We must recognize, 

 then, as distinct cases : — 



1. Original hornblende enclosing augite-grains, without crystallo- 

 graphic relation ; 



2. Perimorphic hornblende (secondary) bordering augite-cores, 

 with crystallographic relation ; and 



3. Complementary hornblende (original) surrounding augite- 

 nuclei, with crystallographic relation. 



One or more of these is found in all the slides examined ; but I 

 have not yet certainly recognized the first and third in the same 

 specimen. This is, perhaps, in accordance with what might be 

 expected ; for we must suppose in the former case a pause between 

 the consolidation of the augite and that of the enclosing hornblende, 

 and such a pause would presumably be unfavourable to the formation 

 of complementary hornblende upon nuclei of augite. 



Another kind of amphibole may also be mentioned, though it is 

 less common here than in some other hornblende-diabases, such 

 as those of Pen-y-rhiwiau, near Clynog-fawr, Caernarvonshire, of 

 Jersey, of Little Knott in Cumberland, and from near Llanerchymedd, 

 in Anglesey*. This is the "secondary enlargement" hornblende of 

 Van Hise t, first described in eruptive rocks by Becke J. It is a 

 growth of hornblende-substance entirely posterior to the con- 

 solidation of the rock, but proceeding in crystalline continuity with 

 preexisting hornblende, so as to border the original crystals, fill the 

 interstices between them, or form a narrow and ragged fringe inside 

 the walls of an included grain of serpentine. Evidently its deposi- 

 tion can only proceed concurrently with the destruction of some 

 other mineral. This later growth of amphibole is clear and colour- 

 less or pale green; it constantly gives rather higher polarization- 

 tints than the brown hornblende : frequently it has the cleavage- 

 traces but little pronounced. Such hornblende, forming a " secondary 

 enlargement" of original and perimorphic crystals, is found in 

 several parts of the district. Becke also describes a similar horn- 

 blende-growth bordering original augite-crystals ; but of this I have 

 found no evidence in the Sarn district, though it is weU seen in a 



* * Geol. Mag.' dec. 3, vol. iv. p. 552. 



t ' Amer. Journ. Sci.' ser. 3, vol. xxxiii. p. 385 (1887). 



I Tschermak's ' Mittheilungen,' vol. v. part ii. (1883). 



