652 GEOLOGY OF SAMOA AND THE ERUPTIONS IN SAVA1I, 



pahoehoe type, though hummocks, bulges, and cracks due to 

 cooling abound. The tachylitic crust shows ropy, wave-like and 

 tapestry-like sti'uctures due to variations in the rate of flow of the 

 plastic lava on the surface, and other causes which have been 

 already explained by Dana.* Curious concentric wrinkles and 

 ripples are often seen; and are in some cases, if not in all, caused 

 where lava flows from the surface into a hole left by the burning- 

 out of a tree by a former flow. Such holes abound, especially 

 where the lava has destroyed cocoanut plantations. Cavernous 

 hollows, with lava stalactites and stalagmites, abound in the 

 scoriaceous crust, and the roof often breaks when a pedestrian 

 walks across. Patches of the flow conform to the Aa type. A 

 narrow patch of this nature has to be crossed in going from 

 Matauto to Satapatu, on the eastern side of the flow. Other 

 patches of Aa exist on the eastern side of the flow, west and 

 south-west of Patumea. 



From the basic nature of the lava and the level surface of the 

 main flows, one would at first suppose that the lava was very 

 liquid, and had come down with great rapidity. Such is, however, 

 not always the case. It flows rapidly into and out of the crater, 

 and probably in the main lava-tunnels; but, as soon as it spreads 

 out in a sheet on the surface, it becomes very pasty, and 

 advances very slowly (in most cases not as rapidly as a man can 

 walk). The reason of this sluggishness is that, owing to the low 

 temperature necessary for the liquidity of a basic lava, the crust 

 cools quickly, tends to hold back the molten lava within and 

 behind, and advances with it like an encompassing filament. 

 The lava front advances, and grows by cracking when the 

 interstices are filled with pasty lava which quickly cools. By a 

 repetition of this process of intercalation, the stream spreads and 

 advances. Stones and rubbish taken up in the lava front also 

 hasten the cooling and impede the progress. In many places the 

 lava advances only a few yards per hour at the time of the great 

 flows. At other places it advanced as fast as a man walks. I 



* Characteristics of Volcanoes. 



