TKANSACTIOXS OF SECTIOX C. 603 



The series lias in Gleucoe a general south-easterly dip, and the higher beds 

 gradually overlap on to the older schists towards the eastern portion of the area. 

 Thus, in the Blackniount the rhyolites rest immediately upon the sehists, the 

 surface of tbe older rock.s being remarkably steep. During the author's recent 

 work for the Geological Purvey the age of these volcanic rocks has been dehuitely 

 settled by the discovery of Lower Old Red plant remains in a bed of black shale 

 at the base of the east fnce of the Buachaille Etive Mor. Iii this bed Messrs. 

 Peach and Tait found specimens of Psilophytou and I'achytheca in June last. The 

 associated lavas are therefore of the same age as those of Lome. 



In the lilackmount the volcanic rocks are cut and metamorphosed by the 

 granite of the Clach Leathad, which is a continuation of that of Ben Uruachan to 

 liie south-west, and of that of the Moor of Kanuoch to the north-east. Again, a 

 large mass of granite, of Ben Cruachan type, is intrusive in the basic andesites on 

 the south side of Glencoe, near Clachaig. Thus, these granite masses are of later 

 date than the volcanic period of Lower Old Red Sandstone times. It should be 

 mentioned in this connection that in the basement conglomerates on tbe north side 

 of Glencoe well rounded boulders of hornblende-granite and diorite occur, and 

 granite blocks have also been found in the breccias near Xingshouse. It is not 

 possible, however, that these boulders could have been derived from the granitic 

 masses now exposed in the same neighbourhood, as these have been conclusively 

 shown to be of later age than tbe volcanic series. 



The final phase of the igneous activity of the district was marked by the forma- 

 tion of numerous dykes of porphyrite and quartz-porphyr}', which are seen to cut 

 both the granites and the lavas. 



4. Notes on the New Geolvgkal Map of Victoria. By James Stirlixo. 



.5. On thf. OriqinaJ Form of Sediment a ry Deponts. 

 By Rev. J. F. Blake, M.A., F.G.S. 



In determining the position and outline of ancient continents by observing the 

 direction in which the sediment derived from them thickens or thins, and in 

 interpreting the significance of the contours of the sea-bottom, it is necessary to 

 consider what will be the actual shape as a whole of any sedimentary deposit of 

 single origin in relation to the land whence it is derived. 



It has been assumed by several authors, boUi of text-boohs and of special 

 memoirs, that sedimentary deposits are thickest near their source of origin, and 

 that limestones are deposited at great distances from tbe shore. The author gives 

 reasons for taking a somewhat different view. The action of tides and currents 

 is considered as a subsequent operation ; the original form of the deposit would 

 be that which would result from a river carrying detritus directly outwards 

 into a tideless sea. The coarse material which is pushed along the bottom of a 

 river fills up the angle between the lowest level the river reaches and the con- 

 stantly deepening bottom of the sea, and the deposit is consequently wedge-shape 

 towards the land and sharply sloping seawards, like the tip of a railway embank- 

 ment. The detritus which floats consists of all that has too low a power of sinking to 

 overcome the velocity of the stream, and only begins to sink on the retardation of 

 that velocity. The detritus with any particular rate of sinking will thus descend 

 faster and also be crowded into a narrower area at a distance from the shore, and 

 so form a thicker deposit there, where alone there is permanently room for it. This 

 result will be modified by (1) the expansion laterally of the retarded stream, 

 (2) the evaporation of the surface-water, (3) the mixture with sea-water, (4) the 

 superposition of various maxima at different distances from shore, (5) the redistri- 

 bution by tides and currents. 



During the continuance of constant physical conditions the seaward boundary 

 of river-brought deposits will thus be a marked line — the * mud-line ' of Dr. Murray — 

 at various depths, according to circumstances. At this line there is a rapid change 



