130 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



reach firmly established conclusions. In certain cases, however, the topog- 

 raphy is so constant in its general features that we feel warranted in 

 drawing conclusions. Certain of the lakes of the State of Alagoas, for 

 example, afford good examples of lakes produced by coast depression. 



The northernmost of these lakes, the Lagoa do Norte, is surrounded 

 on three sides by rather steep-faced Tertiary hills of soft sedimentary 

 rocks. The slopes of these hills are of forty-five degrees and in places 

 even more. 



I learned from Mr. H. Haynes, Superintendent of the Alagoas Rail- 

 way, that a well was driven at the railway shops near the foot of this 

 bluff in the city of Maceio to a depth of two hundred metres, — all the 

 way in loose materials. The inference is that the steep slope of the 

 Tertiary hills continues beneath the surface and that the filled-in sedi- 

 ments are more than two hundred metres in thickness. 



Fig. 07. The deep well section at Maceio and its relation to the rocks of 

 the hills above the city. 



Where the Alagoas railway runs along the lake shore in some places 

 there is barely room for the road-bed between the lake and the hills, 

 while at other places broad meadow-like plains or mangrove swamps lie 

 between the lake and the hills. At still others the line of the road 

 crosses flat-bottomed valleys in passing from one spur of the hills to 

 another. These hills — here steeper and there of gentler slope — extend 

 more tlian three fourths of the way round the lake ' from Maceio at its 

 northeast corner to the east of Coco Seco on the southwest — leaving 

 only the ancient mouth of the bay with a flat low border of sand between 

 it and the sea. 



The lake is now very shallow, so much so that it is navigable even for 

 barccK^as (small boats drawing a little more than a metre of water) only 

 along one channel on the south side. 



Seen from the lighthouse or from the hills -that overlook Maceio, the 

 coastal belt inland is mostly one broad flat-topped table-land dropping 

 off abruptly on the coast. This table-land is made of sedimentary rocks 

 that lie in nearly horizontal beds, laid down during Tertiary times. 



