164 



KNOWLEDGE 



[September 1, 1892. 



was removed from the surface of tbe Mendip Hills, and 

 most of this destruction took place during the above interval. 

 No one has yet attempted to apply a rate of denudation to 

 this case, for the uncertain elements in such a problem are 

 many. The Mississippi rate of one foot in 6000 years 

 would hardly be applicable, being an average for a large 

 area including mountains, valleys, and plains ; whereas 

 the Mendip HUls are a small hilly area. If we could find 

 the rate at which some of our present mountainous regions 

 are being worn down, and obtain an average therefrom, it 

 might be justifiable to apply such an average to the case in 

 point. But mountains are composed of hard and often 

 crystalline rocks, and this fact would tend to counteract 

 the more rapid erosion due to the velocity of mountain 

 streams. 



We will now endeavour to point out a method that might 

 possibly lead to valuable results if followed, and from 

 which an average rate of rock formation might be obtained. 



Take the case of the Mississippi. What becomes of all 

 the soHd matter brought down by this river ? It mostly 

 finds its way into the Atlantic, for the Gulf of Slexico is 

 swept by that powerful cun-ent the Gulf Stream. It 

 would not be spread till unr the Atlantic bed, for some may 

 be carried up to the North Sea and Arctic Ocean ; and 

 again, there are large areas in the Atlantic where no 

 ticdimi ntanj dejjosits are forming, but only globigeriua ooze, 

 pteropod ooze, or the red clay ( believed to be volcanic and 

 even cosmic dust). These areas are far from land, and 

 some of them are the deepest recesses of the Atlantic. 

 Suppose that all the d'hris from the North American con- 

 tinent were being washed into the Atlantic only. Now 

 this o:ean is larger than North America in actual area, 

 but we may subtract the areas devoted to globigerina ooze 

 or red clay. What these areas are could doubtless be 

 estimated by Mr. Murray, of the CliaUciKjer expedition. We 

 do not know how much they are ; but let us suppose that 

 when this is done, an area remains equal to the continent 

 of North America. Then it would follow that all the rock 

 material removed from that surface of laud settles down to 

 form new rocks on an area e<jual to that of the hind from 

 iihich it came. Now if, takuig the Mississippi rate, one 

 foot is removed from the former area in 6000 years, it 

 follows that about one foot is added to the latter surface in 

 the same time. It would really be rather more, because 

 the new material would be soft and imhardened by pressm-e, 

 whQe the old rocks from which it came were compressed 

 and hardened before they came up to form a land smface. 

 But this difference may be neglected. It will thus be 

 seen that a result of some value is obtained, namely, 

 just what we have been seeking — an average rate of rock 

 formation. 



The question arises — " Is this rate of rock formation 

 over a large area of sea bed, viz., one foot in 6000 years, too 

 rapid '? " We are inclined to think that it is. It might 

 apply to strata formed in shallow waters, but it seems too 

 high a rate for those formed in deeper waters, and 

 certainly is inappUcable to slow-growing deposits like 

 globigerina ooze. However, let us see what we can 

 make of it. The whole series of stratified i-ocks is 

 generally estimated at 100,000 feet — taking all the forma- 

 tions and adding their thicknesses together. Here would be 

 a measure of geological time, if only we knew the average 

 rate at which they were built up. Suppose we apply 

 the rate just obtained and see what it leads to. If one 

 foot is formed in 0000 vears, 100 feet wiU be formed in 

 600,000 years, and 100,000 feet in 600,000,000 years. Six 

 hundred millions of years ! This is more than Lord 

 Kelvin's extreme limit for geological time, or the time 

 since the earth consolidated fi-om a molten state. And yet 



we have taken a rate of rock forulation that appears not to 

 err on the side of rapidity ; and, moreover, this calculation 

 makes no allowance for those great " gaps " or " breaks " 

 in the 100,000 feet of the geological record with which the 

 student will be familiar. Again, it makes no allowance 

 for the necessarily slow rate at which organic deposits were 

 formed, and formations of this kind occupy no small 

 fraction of the whole series of rocks. For example, the 

 great moimtain limestone in one district is 4000 feet thick ; 

 the chalk in the Isle of Wight is 1000 feet thick ; then 

 there are the oolites between, to say nothing of Silurian 

 limestones below. 



It is therefore not surprising that geologists are 

 dissatisfied with the limits laid down by Lord Kelvin and 

 others. They demand much more time than he will allow, 

 and we think that the calculation above given justifies 

 such a demand. His later estimate of only 100 millions of 

 years certainly seems too small. Professor Huxley, some 

 years ago, endeavoured as it were to make peace between 

 the two parties in this controversy by taking the latter 

 limit of 100 millions and applying it to the sti-atified rocks. 

 If 100.000 feet of rock were formed in 100 millions of years, 

 then the rate of rock formation would be one foot in 996 

 years — say, roughly, 1000 years. Now the result we 

 obtained above was one foot in 6000 years, so that our 

 rate is six times slower than that which follows from Lord 

 Kelvin's computation, and we venture to think that it 

 would be more acceptable to geologists. 



One cannot help hoping that before long some attempts 

 will be made to observe the rate of deposition in different 

 seas. Would not observations of the amount of sediment 

 suspended in sea-water, taking samples from various 

 depths, be useful ? But it would be better still if someone 

 would let down vessels (like rain gauges) on to the bed of 

 the sea in various spots, leave them there for twenty years, 

 and then take them up and measm-e the amount of solid 

 matter contained in them. They could be attached to 

 buoys by thin wire ropes ; thus the sites would be indicated 

 and they could be pulled up. Or again, perhaps in the 

 future an international committee of scientific men may 

 be formed to observe and measure the amounts of dehris 

 brought down to the }*Iediterranean by all the principal 

 rivers flowing into it ! It would take a long time, but the 

 work could be divided up, and when done we should have 

 a fair idea of the amount of sediment settling down in that 

 area of sea, and so could calculate the rate of rock 

 formation that obtains there. 



BEE PARASITES.-III. 



By E. A. BuTLEK. 

 {Continued from paije 145.) 



THE parasites from which solitary bees suffer belong 

 chiefly to two orders, the Hymenoptera and the 

 Coleoptera. We have already seen how the stores 

 of food they accumulate for their young are liable 

 to be appropriated by cuckoo bees ; we have now 

 to show what persecutions are inflicted upon them by 

 other Hymenoptera, and by some very remarkable beetles, 

 examples of the order Coleoptera. And first as to the 

 former. Some of the most resplendent of all British 

 insects are the ruby-tailed flies, golden wasps, or fire- 

 tails (Fig. 5). They constitute the family Chri/sididii , a 

 carnivorous group of small extent, and not unlike small 

 bees in shape. If they were only of larger size, they 

 would vie with the most gorgeous productions of the 

 tropics in splendour, but small as they are, then- brilliance 



