1848.] SMITH ON SCRATCHED BOULDERS. 17 



G. SEXTANS, Hall, Plate I. fig. 10, 6, c, magnified. 

 Syn. G. sextans. Hall, Paleont. New York, pi. 74. fig. 3. 



Small, thin, rounded at the base, and branched directly from it at 

 an angle of 45° or 50°, and with broad teeth outside ; the depth of 

 the teeth almost as great as the width of the shaft. 



Nineteen or twenty specimens on one slab present the same cha- 

 racters. 



This is a new form, and it is a more simple variation of the two- 

 edged Graptolites than those with teeth on the inner sides, in which 

 the splitting of the axis does not seem so intelligible. Our specimens 

 are very ill-preserved, and were probably very thin. 



Loc. Black slate, west of Wigtownshire. 



3. On Scratched Boulders. 

 By James Smith, Esq., F.R.S.L.&E., F.G.S. 



Part I. 



[Read April 19, 1848.] 



There are two modes by which we may suppose that boulders have 

 been scratched ; they may have been held fast in a fixed position 

 whilst some hard substance passed over them, or they may have been 

 entangled in the under surface of a moving body, such as an iceberg 

 or glacier, and dragged over rocks, which would thus also be scratched. 



I cannot doubt but that both these causes have contributed to the 

 production of the phaenomena in question. The instances to which 

 I mean at present to call the attention of the Society belong to the 

 former class — the boulders have been stationary whilst the scratching 

 body, whatever it was, passed over them. 



In a former communication * I stated, that I had observed, on the 

 shores of the Gare Loch in Dunbartonshire, two boulders half im- 

 bedded in the till or diluvial covering, both of them grooved in the 

 same direction, from N.N.W. to S.S.E., and concluded that it was 

 not probable that the parallelism was accidental : subsequent observa- 

 tions have fully confirmed this conjecture. In the following year Mr. 

 Maclaren of Edinburgh discovered rocks on both sides of the Gare 

 Loch, which were grooved in the same direction as the above-men- 

 tioned boulders : I have since had an opportunity of confirming his 

 observations and of discovering additional instances, some of them in 

 the immediate vicinity of the two boulders. I have also discovered 

 several additional scratched boulders, and in every case the direction 

 of the scratches is the same. As this is also the direction of the axis 

 of the valley which forms the trough of the Gare Loch, Mr. Mac- 

 laren concludes that they have been caused by a glacier, which for- 

 merly filled it. 



"Whatever was the cause, it must have been subsequent to the de- 

 position of the till at least in this locality. We must be careful there- 

 fore not to confound the two phsenomena, and conclude that these 



* Read June 4, 1845. 

 VOL. V. part I. C 



