SECT. 2] 



MICROTOPOGRAPHY 



443 



layers of marine organisms which are often only made apparent by colour 

 jihotographs. The manganese dioxide layer can vary in thickness from a few 

 microns to several centimetres, first, discolouring the rock and, secondly, 

 obscuring the surface texture with a botryoidal crust (Fig. 7). The extent of 

 this is determined by the rate of manganese deposition and the length of ex- 

 posure of the rock face, and may, therefore, be used as a guide to the relative 

 ages of rocks within a given area. 



b. Boulders and cobbles 



Many photographs show areas strewn with boulders and cobbles. These may 

 originate in three different ways. First they may be derived locally by the 



Fig. 8. 34° 53'N, 16° 32'W. Depth 1550 m. Near the peak of a seamount north of Madeira. 

 Area of picture 3 by 2 J m. (Photo by N.I.O.) 



Boulders of volcanic basalt about 30 cm across partly buried by Globigerina ooze 

 with a thin coating of manganese. The camera is looking into the side of the seamount. 



breaking up of bedrock by some submarine weathering process : in the case of a 

 cooling lava flow, the breaking up may take place very shortly after solidifica- 

 tion as a result of internal stress set uj) by uneven cooling. Secondly, they may 

 be transported to the area by some agency such as ice rafting or ejection from a 

 volcano. Thirdly, they may have grown in situ by the accumulation of chemical 

 or biochemical deposits to form, for example, manganese or phosi)horite 

 nodules. 



Typical boulders produced by the fragmentation of solid rock are shown in 

 Fig. 8 which are highly vesicular basalts on the sides of a seamount. Parallel 



