18 PROF. T. G. BONNEY ON THE HORNBLENDIC AND OTHER 



clase, apparently oligoclase, is very abundant ; but I think that or- 

 thoclase is also present. The hornblende is of a pale green colour, 

 moderately dichroic, with irregular outline and characteristic cleav- 

 age. The mica (in small flakes) is of a red-brown ; some granules 

 of an iron oxide are present. The largest (haematite ?) encloses a 

 small, imperfectly formed crystal of hornblende ; microliths of horn- 

 blende and mica occasionally occur towards the edges of the larger 

 felspar crystals, and various microliths, some probably needles of 

 hornblende, others possibly apatite, occur in the ground-mass of the 

 slide. 



Specimens of the most granitic varieties of this series were de- 

 scribed in my last paper (vol. xxxiii. p. 886), and I have examined 

 a rather more compact specimen from a lenticular band near the 

 W. headland at the Balk Bay ; but in the case of this, except that 

 the materials are more minute, the structure is so similar to the 

 others that no additional description is needed. 



The last to be described is a specimen of a rock underlying the 

 projecting ridge on the rocky shore in Polbarrow Cove, whose 

 weathered exterior much resembles an ordinary indurated felspathic 

 and micaceous grit, an appearance which is not wholly lost on freshly 

 broken surfaces. It consists of felspar, brown mica, and horn- 

 blende, with some quartz, the last generally in small grains and 

 sometimes apparently interstitial. The description already given of 

 the minerals will suffice ; but the slide exhibits a more uniformly 

 granular character than do the others. 



D. Remarks on the Structure of these Groups. 



I have discussed at some length the structure and mineral character 

 of these rocks because they give rise, in my opinion, to some consider 

 rations which may prove to be of a little importance. Examina- 

 tion in the field and with the microscope fully justifies us in 

 classing this last group as well as the others with the metamor- 

 phic rocks, and even among those which have undergone a very 

 great amount of alteration. Many of the mica-schists in the lowest 

 group, many of the hornblendic rocks in the middle one, and not a 

 few of the granitoid bands in the upper are evidently highly crys- 

 talline. Their original structure has disappeared. It is a matter 

 of conjecture what was the nature of the sediment of which they 

 were formed, and to this their chemical composition alone can afford 

 a clue. Yet occasionally, especially in parts of the middle and 

 upper groups, we have the most minute phenomena of stratification 

 clearly recorded by the mineral structure. It can hardly be doubted, 

 I think, that such a rock as the banded epidotic hornblende schist of 

 the headland in Cadgwith Cove, or that with the wonderful current- 

 bedding at Hot Point, or such as those which abound in the upper 

 group, has been the result of an original clastic structure. It seems, 

 then, possible to me that in these and in some of the curiously 

 banded rocks in the upper group many of the constituents may be 

 in part original. I do not mean that any one grain as it now stands 



