126 THE HABITAT OF THE EURYPTEREDA 



been gleaned from a detailed study of many more sections than can 

 be mentioned here, lead to the conclusion that there was no erosion 

 between the deposition of the last of the Llandovery (Birkhill) beds, 

 whether they were Lower or Upper Birkhill, and the lowest Tarannon 

 beds. The structural relation is, therefore, one of replacing overlap, 

 the Tarannon beds pushing to the southeast just as rapidly as the 

 graptolite-bearing Llandovery muds retreated in the same direction. 

 Lines of sections at right angles to the strike of the Llandovery arid 

 Tarannon rocks taken in various places from coast to coast, indicate 

 that in the northern part of the central belt the Tarannon is always 

 of a massive unfossiliferous character, grading southeastwards into 

 graptolite-bearing shales and mudstones. There is no doubt that 

 the coarse conglomerates and grits were river-borne. In one of the 

 typical localities in the Moffat district Peach and Home, in describ- 

 ing the conglomerate say, "the rock possesses a greywacke matrix, 

 in which are embedded rounded pebbles of quartz, red chert with 

 radiolaria, Arenig volcanic rocks, with boulders of granite and quartz- 

 ite from eight to ten inches in diameter. Some small pieces of mica 

 schist have also been observed. The fragments of quartzite and mica- 

 schist resemble rocks of those types in the Eastern Highlands; there 

 can be little doubt that they were derived from that region" (P. 

 and H. 215, 210). These authors also note in regard to the grey- 

 wackes and grits that, "both volcanic and plutonic rocks have con- 

 tributed to their formation. The fragments are angular or sub- 

 angular. Well-rounded grains are rare. There is, further, a very 

 great variability in the sizes of the constituent grains; indeed, the 

 material does not appear to have been well sorted by aqueous action" 

 (215, 211). It seems surprising that materials which had been trans- 

 ported by rivers for so great a distance, it being about one hundred 

 miles from the Eastern Highlands to the Moffat district, should not 

 be better sorted. However, it is clear that the material must have 

 been brought down by rivers. That it was deposited as a subaerial 

 delta or series of deltas which spread out into the sea to the south- 

 west is suggested by the character of the materials, for in the extreme 

 north of the Tarannon area there are only the coarse conglomerates 

 without fossils, but these deposits merge ever so slowly into finer ones 

 southward, the first change in the conditions of sedimentation being 

 indicated by the intercalation of thin, leaf-like beds of shales bearing 

 graptolites. Indeed, this domination of terrestrial over marine sedi- 

 ments is seen even towards the close of the Llandovery in many 



