826 MR G. W. TYRRELL ON 



and mudstones consist of an excessively fine quartz-flour, mixed with greenish 

 sericitic mica, black opaque material which is partly iron-ores and partly carbonaceous 

 matter, and much indeterminate material. A greyish granular substance, identified 

 as leucoxene, is also present. The granularity of the material varies from a flour-like 

 fineness to a comparatively coarse grade, at which point the rock passes into the 

 argillaceous grits described above. Many of the rocks are finely banded with alter- 

 nations of coarser and finer material. The coarser types are sometimes abundantly 

 radiolarian. Some of the shales are largely composed of tuffaceous material, as 

 is shown by the presence of felspars and sporadic scapolitisation (see postea, 

 p. 829). 



The first effect of pressure on these rocks is to induce a slaty cleavage by the 

 alignment of the minute constituent particles. At this stage there is little or no new 

 mineral formation. With the advent of greater pressure and shearing stresses the 

 cleavage laminations are closely crumpled and puckered, and are finally ruptured 

 along parallel planes so as to produce a good strain-slip cleavage. The planes of the 

 latter are frequently so close as to produce a new fissility which entirely obliterates 

 the older. There is now considerable new mineral formation. The argillaceous 

 material of the groundmass is transformed into large flakes and films of sericite. 

 Ferriferous and other material has gone to produce clots, folia, and isolated crystals 

 of a yellowish-green chlorite which shows good " ultra-blue " polarisation tints. The 

 puckering and strain-slip is outlined by carbonaceous material (see fig. 1, Plate XC1V). 

 These rocks are phyllites or slates in which there has been considerable new mineral 

 formation. A further stage of metamorphism would produce mica-schist. The 

 phyllites are thoroughly permeated with thin quartz veins which have suffered both 

 puckering and strain-slip along with the other constituents, and therefore antedate 

 the crushing. This series of rocks doubtless corresponds to Thurach's thonschiefer 

 and phyllit. 



The calcareous rocks in the collection are few in number. Some of them are 

 described later as calcareous tuffs. These consist of volcanic fragments embedded 

 in a calcareous paste. Others may be described as calcareous grits. They contain 

 numerous angular chips of quartz and felspar (oligoclase), with fragments of chert, 

 shale, fine-grained grit, and trachyte, in a paste of muddy calcite mixed with crypto- 

 crystalline siliceous matter. The bedding is outlined by wisps of carbonaceous 

 material. Other types are devoid of rock-fragments save a few of trachyte-glass. 

 These have faint traces of organic remains, which are, however, quite unrecognisable. 

 Some of the calcareous types have suffered kataclastic deformation along with the 

 other sedimentary rocks. A very fissile limestone-slate, from the lower division of 

 the Cumberland Bay Series, occurs in the collection. Others are much crushed, and 

 veined with quartz. In thin section one of these rocks is seen to be recrystallised 

 into large mutually interfering plates of calcite, which show the bent, confused, and 

 discontinuous twin-lamellation characteristic of strain. One of the phyllites of 



