PRESENT CHANGES IN THE EARTH. 



95 



small and large, and such shells as are now met with in 

 the lake. 



177. Deltas. The name delta is commonly given to 

 the accumulation from a river at its mouth as it empties 

 into a sea or the ocean, when it has gone so far as to form 

 flats, through which the river runs usually with a net- 

 work of channels. The name is given from the common 

 resemblance in the form of this accumulation to the Greek 

 letter A, which is called Delta. The shape which the de- 

 posit tends to take is fairly represented by Fig. 36. From 



Fig. 36. 



the point where the river enters the sea the sedimentary 

 matter spreads out or radiates, and at the same tin>e it is 

 tossed about and beaten back by tides and the waves. It 

 is this action of the sea, in opposition to the prolongation 

 of the river current into it, that produces the bars so often 

 found in such cases. These bars are indicated in the fig- 

 ure at the sea-extremity of the delta. The numerous sand- 

 bars formed at the mouth of the Mississippi render the en- 

 trance to it difficult and hazardous. Sometimes even isl- 

 ands are formed. Thus, about fifty years ago, an island 

 was formed opposite the mouth of the Hoogly River, 

 which in 1818 was two miles long and half a mile wide, 

 was covered with vegetation, and was inhabited ; but 

 afterward this was gradually swept away, and dwindled 

 to a mere small sand-bank. It is common there, and in 

 other cases also, for islands to form, and then be de- 

 stroyed by the change of currents from the formation 



