DRIFTS OF THE TALE OF CLWTD. 



105 



as they dovetail into one another, and lines of red sand seem often 

 to have been washed far out on to the growing travertine. Where 

 this drift overlaps the limestone it must be washed into the crevices 

 and caves which are being formed, as the result of the chemical 

 solution of which we see the proof in the deposit of calcareous tufa 

 at Pwllgywn *. 



Part VI. 

 The Cave-deposits. 



The age and manner of formation of the deposits in the caves is 

 quite a separate question from that of the age and manner of for- 

 mation of the caves themselves. 



As a general rule, we may say that the time of formation of caves 

 was a time of destruction. Ever increasing streams were rushing 

 into and through the cave as it was being enlarged, and of course 

 but little deposit of that date could permanently remain. It was 

 when the cave had been deserted by the streams that formed it 

 that the age of accumulation of cave-deposits began. Por a long 

 time it was still subject to overflow and flood, and the earlier 

 deposits always ran a great chance of being swept out. 



Such a cave, brought within reach of the action of the sea, would 

 be soon cleared out, and every tide would swill it out af v esh. 



When we try to fix the date of cave-deposits we must appeal to 

 the same kind of varied evidence as that on which we base our 

 classification of the sedimentary rocks in a natural system. 



We must from an examination of the district try to make out 

 when it was that the local conditions first allowed any deposits to be 

 laid down in the cave, and establish their true order of succession. 



"We must also examine the paheontological evidence. The 

 deposit may be very recent, yet contain remains not now common 

 in the area from which it was derived, as for instance when a district 

 has been cleared of wood, and the land-shells and other forms of 

 life have changed in consequence, the cave-deposit may tell the 

 story; we should find those forms of life that haunt the woodland 

 succeeded by those that love the open ground . Without taking such 

 circumstances into account, and making due allowance for habit and 

 habitat, percentage is a very unsafe test in palaeontology ; still, when 

 we are dealing with bone-caves in which great beasts of prey have 

 gathered the remains of all the animals they fed upon, we have the 

 record of many of the forms of life that do not usually frequent 

 caves, and we can compare the list with those drawn up from all 

 the bones found in the ancient river-gravels and old marine deposits, 

 and can tell whether they most resemble the older or the newer 

 groups of life. In this way sufficient evidence has been collected 

 to form a rough chronology to which we can appeal in isolated cases 

 for a date. 



Now let us turn to some of the caves in the same district, and 

 see what we can learn from an examination of their contents. 



* See Maw, Geol. Mag. vol. iii. p. 253. 



