304 A. GEIKIE ON THE SUPPOSED 



of the Cambrian period possessed such a composition, however, is put 

 beyond question by the analyses of the basic tuffs already given ; so 

 that it is not necessary to endeavour to discover a late and intrusive 

 origin for the sheets of Rhosson and Clegyr Foig. 



I have examined under the microscope thin slices taken from the 

 rock at both the localities just named, also from the crag south of 

 Castell, and from the cliffs at the southern end of the promontory 

 between Ramsey Sound and Pen-y-foel (Plate IX. fig. 4). In all of 

 these the general composition is alike. There is a variable quantity 

 of a base, which under a -j- objective is resolved into ill-defined coales- 

 cent globulites and fibre-like bodies, which remain dark when rotated 

 between crossed nicols. In some varieties, as in part of Rhosson 

 Crag, the base is nearly lost in the crowd of crystalline constituents ; 

 in others, as in the crag south of Castell, it forms a large part of 

 the whole mass, and may be seen in distinct spaces free from any 

 crystalline particles. Through this base are diffused, in vast num- 

 bers, irregularly shaped grains of augite, seldom showing crys- 

 talline faces with measurable interfacial angles. These grains, 

 or granules, may perhaps average about 0-003 inch in diameter. 

 Plagioclase is generally hardly to be recognized, though here and 

 there a crystal with characteristic twinning may be detected in the 

 base. Magnetite occurs abundantly — its minute octahedra, with 

 their peculiar colour and lustre, being apparent with reflected light 

 on the fresher specimens, though apt to be lost as diffused ferrugi- 

 nous blotches in the more decomposed varieties. But perhaps the 

 most remarkable ingredient is olivine. I have referred to the red 

 hEematitic crystals which, even to the naked eye, are visible, dispersed 

 through the ground-mass of these rocks. With a lens these may be 

 observed to be orthorhombic in form and to be evidently pseudo- 

 morphs after some mineral which has been converted chiefly into 

 hfematite. I have often noticed red pseudomorphs (ferrite, as they 

 have been called) in Carboniferous and Old Red Sandstone porphy- 

 rites, where in some cases they appear to be after hornblende, and 

 in others after augite, but occasionally are suggestive of olivine, 

 though with no trace of the original substance of that mineral. In 

 the lava associated with the tuffs at the south end of the promontory 

 between Ramsey Sound and Pen-y-foel, however, I find some large, 

 well-developed pseudomorphs, which are certainly after olivine. They 

 have the characteristic contour of that mineral and its peculiar 

 transverse curved and irregular fractures. The average length of 

 these pseudomorphs was found, from the measurement of six examples, 

 to be 0-023 inch, the largest being 0-034, and the smallest 0*014. 

 Seen by transmitted light they present a structureless pale-green 

 material nearly inert in polarized light, round the borders and 

 across fissures in which an opaque substance has been developed, as 

 serpentine has been in the familiar alteration of olivine. The pale- 

 green matter may be the result of a first alteration, subsequently 

 replaced along the borders and across the fissures by the dark 

 substance. With reflected light the latter is found to be bright brick- 

 red. It is evidently chiefly haematite. Every stage may be traced, 



