548 Prof. T. 0. Bonney — North- Western Gharmuood Forest. 



reddish. The difference also between fragments and matrix is often 

 less marked under the microscope than it appears t'o the unaided eye, 

 and in some slices it is almost impossible to determine the precise line 

 where the one ends and the other begins. The structure of both 

 occasionally" approaches microgranular, but is more often intermediate 

 between the Peldar and the Sharpley types, inclining on the whole 

 to the latter. In fact, the relationship between these porphyroids 

 and the brecciated Bardon rocks has of late impressed me more 

 strongly than ever before. Certain specimens may present characters 

 which seem to be distinctive, but on studying others we find these 

 gradually disappear, 1 the Peldar ' spotting ' being interstreaked with 

 a Sharpley 'speckling' as if the two varieties were imperfectly 

 mingled. At the Tor quarry a rather compact purplish porphyron!, 

 like that at Bardon, appears to be intrusive in the Peldar porphyroid, 

 and the former shows by the arrangement of its microliths a slight 

 fluxional structure ; but, as in the other locality, the difference 

 between the two porphyroids is more conspicuous to the unaided eye 

 than under the microscope. The compact porphyroid, however, 

 sometimes has a greenish tint, and at Bardon these colour varieties 

 may be seen irregularly interbanded, the one passing into the other. 

 Even the Peldar porphyroid at the Tor quarry sometimes shows 

 a purplish ' bloom ' ; in fact, I am now convinced that the colour 

 differences in the north-western district largely depend on the 

 subsequent action of water, which has converted some of the more 

 minute constituents into viridite. 2 



The large quarry on Peldar Tor is rudely crescent-shaped, and has 

 been worked back from the road towards the rugged summit of the 

 hill, being carried to a greater depth on its western side. The 

 intrusive compact porphyroid sometimes makes sharp junctions with 

 the Peldar porphyroid, but sometimes partly melts it down, so that 

 the relics of the latter produce a sort of streaking or local mottling 

 in the former. Running obliquely up the wall of the quarry, on its 

 north-western side, is a ' greenstone ' dyke, the thickness of which 

 cannot easily be estimated owing to the direction of the section, but 

 perhaps a couple of yards would not be far wrong. It has evidently 

 suffered much from crushing, the effects of which are not nearly so 

 perceptible in the porphyroid. A slice cut at right angles to the 

 shear planes shows under the microscope (1) streaky patches of 



1 Signs of fluxion, as well as of subsequent pressure, may sometimes be 

 noticed, and the former seem to be more common in the neighbourhood of 

 a junction. Many of the Pre-Cambrian and Ordovician felstones, in which 

 some amount of fluxion is perceptible, show, in a single slice, appreciable 

 variations in their microscopic structures. 



2 Viridite is an old term which I think might well be retained. Of course it 

 is vague, but so is the thing thus denoted. It often is obviously made up of 

 tiny flakes, but not seldom has hardly any effect on polarized light (perhaps 

 because they are so minute). Some of the larger flakes appear to be a 

 chlorite, while others have more resemblance to a greenish mica ; others may, 

 perhaps, be serpentine. In fact, I believe it to be often more or less of 

 a mixture, varying in composition with the mineral which has been replaced ; 

 the above-named colour change from purple to green meaning the formation 

 of a hydrous iron silicate. 



