33 



we can see a cause for a break in this continuity of our deposit which 

 might easily introduce an error of hundreds of thousands of years. I may 

 here mention that the case of the Nile is rather an exceptional one, as few 

 rivers are at present known to overflow their banks and jdeld a deposit of mud 

 with the same regularity as it does, and therefore of course the reasoning which 

 applies to the Nile would apply with much greater force to most other rivers. 

 Let us now take a brief glance at the delta of a river, which is where the chief 

 soil forming operations go on. Here we have a very fair means of judging by 

 observing the character of the deposit, whether the river has been at various times 

 a swift or a sluggish one, as a swift river will deposit stones and pebbles on its 

 delta, as for instance the Rhone, and a sluggish river sand and mud, as for instance 

 the Thames, but we have no means of testing how much the river during its 

 periods of greater velocity may have interfered with its action during its periods of 

 lesser velocity, and hence any deductions as to length of time required to produce 

 what we now find in any particular deposit would be exceedingly inaccurate. 



Another important source of disturbance in any calculations as to geo- 

 logical time arises from the alteration of the course of ocean currents. This 

 alteration may arise from various causes, but one of the most important of such 

 causes is the opening up of a new strait or channel between two seas, which wovild 

 immediately give rise to a current if there was any difference between the 

 temperatures of the two seas, such as for instance between the Mediterranean and 

 Red Sea. Now, suppose a now ocean current to arise from some cause or other, 

 and to sweep past the mouth of a large river where a delta had been forming for 

 thousands of years, the effect would be, that if the ocean current were at aU a 

 powerful one, the portion of the delta still under water would be rapidly swept 

 a,way, and in future geological time leave no trace of its existence, and hence no 

 data to tell us of the immense period during which it was in process of formation. 



Once again, another process of rock formation which can be observed in the 

 present day is that of coral reefs. Now, it can be discovered by observation what 

 is the amount of work done by these little coral polyps during certain periods, and 

 therefore some kind of rough deductions can be made as to the length of time 

 required to build up a certain thickness of such coral rock ; but here also we must 

 be prepared to recognise as a most important possible source of error in any of our 

 calculations as to time, the fact of upheaval and subsidence, which processes are 

 all but universal in coral forming districts. The coral insect or polyp cannot 

 work either out of the water, or in water more than 90 fathoms deep, and therefore 

 when a coral reef formed on any sunken island or rock subsides beneath that 

 depth the formation of coral ceases, and this may be the case dxuing immense 

 periods of time, and again when upheaval takes place the coral polyps would 

 again begin their work as soon as they found the rock in a suitable depth of 

 water. In after geological ages, when this rock was observed, no record would 

 be found of the period of time during which a cessation of work had occurred. 

 Here, again, we are obliged to acknowledge how uncertain and unreliable are any 

 conclusions as to geological time which we may come to. 



