DESCRIPTION OP THE FORMATIONS 367 



"In this cornstone series cornstones occur sometimes as continuous beds of 

 pale red or green compact limestone, and sometimes as nodular concretions in 

 the marls ; they consist entirely of amorphous carbonate of lime enclosing 

 some sand, and no fragments of organisms except obscure plant remains can 

 be seen in them. They may owe their existence to the agency of some lime- 

 secreting alga?, like that which forms the 'sprudelstein' of Carlsbad (described 

 by F. Colin in 1862) and the travertine of the Mammoth Hot Springs in the 

 Yellowstone Park, U. S. 



"The Cornstone series has yielded fish of the genera Pteraspis and Cepha- 

 laspis, and there can be no doubt that it is homotaxially the equivalent of the 

 marine Lower Devonian and of the Lower Old Red Sandstone of Scotland. 

 The Senni Beds are included in this series, because they also contain Pteras- 

 pis; they only occur in that part of the area which lies to the west of Breck- 

 nock, where they have a maximum thickness of 1,200 feet. . . . 



"When the formation is followed westward beyond Llandeilo several changes 

 are found to take place. The base of the Cornstone series ceases to show an 

 upward passage from the Tilestones. the basement beds becoming first sandy 

 and then conglomeratic, at the same time gradually passing across the passage 

 beds and the Ludlow mudstone till, near Carmarthen, they have overstepped 

 the whole Silurian series and rest directly on the Bala Beds. The whole group 

 also becomes thinner, and is not more than 2,500 feet near Llandarog." -* 



These Welsh deposits have been regarded by British geologists as fresh 

 water in origin and formed in a large lake. Much of the formations may, 

 however, equally well, if not better, be regarded as nuviatile. The great 

 thickness, taken with the arenaceous character, indicates shallow-water 

 conditions maintained by subsidence. It would be difficult to conceive of 

 a continuous fresh-water lake existing throughout and excluding the sea 

 which lay not far to the south of the narrow intervening axis of elevation. 

 If, however, sedimentation were, on the whole, faster than subsidence, 

 nuviatile plains would result and the sea could be readily excluded, at the 

 same time that intermittent lacustrine conditions could exist. Further, 

 although the limestones, where pure and thick, suggest such lacustrine 

 conditions, nodular layers of earthy limestones do not require such an 

 origin. It may be concluded, therefore, that in the Welsh basin the de- 

 posits suggest combinations of nuviatile and lacustrine conditions. The 

 large difference which this means in interpretation is that much of the 

 sediment was deposited on river floodplains, and on paleogeographical 

 maps the area should not be represented as a great lake basin, but as land 

 instead of water. 



Turning to the Caledonian basin, this, under the interpretation of 

 Jukes-Browne, may be taken as including all that broad region south of 

 latitude 57° and north of 54°. The chief area is that of southern Scot- 

 land, but small outlying remnants occur in both Scotland and northern 



24 Jukes-Brovrae : The building of the British Isles, 1911, pp. 108, 109. 

 XXVII — Bull. Geol. Soc. Am., Vol. 27. 1915 



