478 PROF. T. G. BONNET AND MAJOK-GEN. C. A. M^MAHON 



that afterwards, as the temperature gradually fell, the whole mass 

 became crystalline *, Thus the banded gneissoid rock of the 

 Granulitic Group is an example of a kind of flow-structure on a large 

 scale, wholly or (more probably) in part antecedent to crystalliza- 

 tion. As this rock, in its distinctive characters, agrees with a large 

 number of " banded gneisses," in which the ordinary symptoms of 

 pressure-modification cannot be detected, and which appear to have 

 at any rate completed their crystallization in situ, this hypothesis 

 may prove to be of wide application. If, however, the bands do not 

 differ very materially in their mineral composition — as is often the 

 case with the banded Archaean gneisses — the hypothesis may assume 

 a simpler form, and with them it may be only necessary to suppose 

 that, as in the case of many rhyolites, some differentiation of consti- 

 tuents had been set up in the magma, the one part becoming slightly 

 harder than the other, though still capable of being drawn out, so 

 that the whole mass assumed a coarse fluidal structure, and subse- 

 quently, since its environment was widely different from that of a 

 normal igneous rock, took on a holocrystalline, yet still a peculiar 

 structure, different from that usually found in granites and diorites. 



(2) The Hornhlenclic Group. 



That this group underlies the Granulitic, is, in most places, a 

 probability rather than a certainty, the two commonly being sepa- 

 rated by a fault or a mass of serpentine. But, assuming the 

 Granulitic Group to occupy a definite horizon, its relations to the 

 Hornblendic are suggested in more than one place, and are, we think, 

 clear in the crags on the south side of Cadgwith Cove, where the 

 latter rocks may be seen gradually rising up from beneath the former. 



The Hornblendic Group exhibits structures curiously imitative, if 

 not actually indicative, of stratification, certainly over a larger area 

 and probably through a greater thickness than the Granulitic t. It 

 includes fairly well banded schists almost everywhere from Porth- 

 alla to the Lighthouses on the one coast, and from Polurrian Cove 

 to near Old Lizard Head on the other. Their structures have been 

 described by the present authors, who have regarded them, though 

 from somewhat different points of view, as indicative of stratifica- 

 tion in the original materials. 



This group has been again examined with considerable care. 

 From the chemical analysis as well as the mineral composition it 

 seems clear that its rocks must originally have been of igneous origin ; 

 the more massive may represent altered basaltic lavas, the more 



* Probably it was a mixture of crystalline grains and half melted stuff 

 rather than a true liquid, so that it was difficult for any mineral to assume an 

 idiomorphic form. The larger porphyritic crystals in the diorite were probably 

 antecior to the epoch mentioned above. 



i' The hornblende-schists are displayed, practically unbroken, in cHffs some 

 200 feet high, between points nearly a mile apart as the crow flies ; the Granu- 

 litic Group, so far as we remember, seldom occurs without a break from top to 

 bottom of such a cliff, or for more than a few dozen yards at most. 



