12 PROP. T. G. BONNEY ON THE HORNBLENDIC AND OTHER 



the southern district, where we have the longest succession of the 

 hornblendic series, though not the largest area, the dominant strikes 

 vary from W.N.W. to N.N.W., and so may be described as roughly 

 N.W. to S.E. This continues as far as the neighbourhood of Cadgwith, 

 after which the strikes are more discordant ; but on the western coast 

 we have the same tendency to a N.W. strike, with now and then, 

 as in the other regions, a deviation to the eastern side of north. Sir 

 Henry De la Beche's record over the northern area of the hornblende 

 schist gives a general W.N. W. strike, so that we may pay less atten- 

 tion to the observations near Porthalla, and say that the normal 

 strike of the metamorphic series is roughly N.W.-S.E., the beds 

 ascending in the easterly direction. Taking 1| mile (6600 feet) as 

 the breadth of the series exposed in the southern area, measured 

 from S.W. to N.E., and 30° as the average dip, the maximum thick- 

 ness of the whole series would be 3300 feet, of which rather less 

 than three fourths would belong to the hornblendic group, and 

 perhaps 300 feet or so to the uppermost group ; but this total 

 estimate may well be cousiderably in excess of the truth. 



III. Microscopic Structure of the Metamorphic Series. 

 A. Micaceous Group. 



This group, as has been already said, is well marked in its litho- 

 logical characters, consisting chiefly of a number of somewhat com- 

 pact dull-green schists, often with a rather " satiny " lustre, com- 

 posed evidently of very minute constituents, moderately fissile, with 

 sometimes a slightly flinty fracture, not seldom very beautifully 

 corrugated. With these are associated occasionally very distinctive 

 mica-schists, and others that are hornblendic, the latter mineral, 

 however, being a dull green, and not the black lustrous variety com- 

 mon in the group above. As will be seen, the green constituent 

 appears very generally to be a variety of hornblende rather than a 

 chlorite (indubitable specimens of which do not seem common) or a 

 mica; but as the latter mineral is not seldom present, and pre- 

 dominates in some bands, I have given this distinctive name to the 

 group. 



Several specimens have been examined microscopically. I will 

 describe first a series collected at the Quadrant, taken in ascending 

 order. (1) A dull green schist, mottled with ashy white (p. 3). It 

 is a distinctly foliated rock, the green constituent being, for the most 

 part, a hornblende in minute rather irregularly formed prisms, with 

 a green mica, probably an altered biotite, and a white mica; asso- 

 ciated with the latter is a peculiar mineral of rather fibrous irre- 

 gular structure, possibly sillimanite. There is a fair amount of quartz 

 in parts of the slide. The whiter patches consist of a granular mineral, 

 composed of minute aggregates, whose nature I cannot determine, 

 with, in parts, much ferrite. Possibly it replaces a felspathic con- 

 stituent. (2) A greyish rock of somewhat gneissose aspect (p. 3). It 

 consists of quartz in granules of irregular outline and very variable 

 size, but rarely exceeding *02", about the same quantity of grains of 



