480 DR. A. JOWETT ON THE [Oct. 1913, 



(2) The composition of the amygdales in the sedi- 

 ments. — The minerals filling the amygdaloidal cavities in the 

 sediments are practically the same as those ibund in the lavas. 

 Calcite is perhaps the commonest, and under the microscope can be 

 seen to have grown from the sides of the cavities as little dogtooth 

 crystals, the rest of the interior of the cavity being then one mass 

 of calcite in crystalline continuity. The cavities are often lined, 

 and sometimes filled, with green chloritic material, in fibrous 

 spherulitic aggregates, and sometimes with a green pleochroic and 

 highly doubly-refracting fibrous mineral. Opal, chalcedony, and 

 quartz have also been noted, and the amygdales are occasionally 

 stained red with iron-oxides. 



(c) Metamorphism of the Sediments. 



The most characteristic secondary mineral produced in the 

 sediments by thermo-metamorphism is brown to golden-yellow 

 in thin section, possesses perfect cleavage, and extinguishes parallel 

 to the cleavage. It is slightly pleochroic and very strongly doubly 

 refracting. It has no definite crystal-outline, only occurring in 

 small, roughly square sections. It most closely resembles astro- 

 phyllite. The mineral is present in quantity, roughly parallel to, but 

 •away from, the contact-zone in the nodules of sediment enclosed 

 in lava, and also occurs in other sections of baked sediment. 



A rude foliation, due to the development of secondary mica in a 

 narrow zone parallel to the surface of contact, occurs in one of the 

 sediments in contact with a basalt. The basalt itself has a thin 

 selvage of tachylyte with a few felspar-microliths. 



V. Pat/lt- Breccias. 



The most striking example is that of the Rock of St. Skae, which 

 ■clearly owes its power to resist the elements more to its reinforce- 

 ment by a multitude of veins of quartz, than to the dyke-rock 

 that apparently forms its core. 



The material filling the north-and-south fractures is seen, under 

 the microscope, to consist of ferruginous chalcedonic silica and 

 •quartz, with a curious banded effect simulating the flow-structure 

 of rhyolites. The transverse veins are obviously more recent, as 

 they break across the others, in some cases binding together a 

 breccia in which fragments of the older quartz-veins are included. 

 Their silica is entirely cryptocrystalline. 



The excessive alteration of the volcanic rocks adjacent to the 

 fault suggests at once a probable source of the silica in the veins. 



Other examples may be seen about a quarter of a mile west of 

 Montroseness Lighthouse, east of Mains of Usan, and on the south 

 side of Lunan Bay. 



A dyke-like fault-breccia occurs a quarter of a mile west of 

 Boddin Point, in which calcite, as well as silica, is deposited in the 

 Tcin. 



