ON THE CRYSTALLINE ROCKS OF THE LIZARD DISTRICT. 497 



by secondary hornblende in igneous rocks would seem to indicate 

 that, in some cases at all events, partial refusion near the earth's 

 surface may account for the formation of such an amphibolite. 

 Hence one observer * has remarked : — " Where the composition of 

 both minerals [viz. augite and hornblende] is identical, temperature 

 alone is sufficient to determine which crystalline form is assumed." 

 AVithout wishing to dogmatize on a matter regarding which our 

 information is at present imperfect, the authors think that the slaty 

 inclusion affords good prima facie evidence that the gabbro, after 

 it caught up the fragment of slate, was never in a highly heated 

 condition. 



YIII. Summary op Results. 



The chief results of the investigations described in this paper may 

 be briefly summed up under the following heads : — 



(1) That the Hornblendic and Granulitic Groups, whatever their 

 genesis may have been, were substantially in their present con- 

 dition at the time when the rock, which is now a serpentine, was 

 intruded. 



(2) That this rock was formerly some variety of peridotite — dunite, 

 saxonite, Iherzolite, &c., occasionally a picrite t ; that the foliated 

 or banded structure, which is perceptible in it in certain districts, 

 does not result from pressure posterior to solidification of the rock- 

 mass, but from movements in it while it was still in a molten or 

 partially molten condition. 



(3) That the foliated or banded structure sometimes present in 

 the gabbro does not result from pressure subsequent to the solidifi- 

 cation of the rock, but it also is a kind of fluxional structure, due 

 probably to movements when the rock was in a condition of rather 

 imperfect fluidity, and consisted of a mixture of crystals and of a 

 magma more or less viscid. 



(4) That the Granulitic Group consists of at least two distinct 

 rocks, one acid, the other basic, of which the former was intrusive 

 in the latter, but that, either in consequence of this or from some 

 other cause, the temperature of the whole mass became sufiicientlj'- 

 elevated in certain localities to allow of movements as in the last- 

 mentioned cases, which have produced the remarkably uniform and 

 stratified aspect of the two varieties ; this movement being followed by 

 crystallization, or completion of crystallization, in the constituents. 



(5) That the Hornblendic Group consists in part of igneous rockf" • 

 that it may be indebted for its structure partly to movements anterior 

 to consolidation, partly to pressures of later date, but that it is difficult 

 to explain all the phenomena either by the one or the other cause, 

 so that at present the possibility of some portions having resulted 

 from the alteration of a stratified basic ash must not be left out of 

 sight. 



* G. H. Williams on Baltimore Gabbros and Diorites, EuU. U.S. Geol. Surv. 

 vol. iv. (188B) p. 46. 



t Viz. an augite-olivine or hornblende-olivine rock, inwhich a small and ratlier 

 variable proportion of felspar or an aluminous silicate is present. 



Q. J. G. S. No. 187. 2 m 



