824 



C. SKOTTSBERG 



With regard to Masafuera, Quensel concludes that "rocks of more normal 

 basaltic composition, principally feldspar basalts, occupy the lower and intermediate 

 elevations of the island", up to approximately looo m; these rocks are mostly 

 vesicular to slaggy, but more compact higher up, "whereas the highest parts 

 consist of an olivine basalt, supersaturated with iron oxides" (1. c. 71). This last- 

 mentioned kind seems to be restricted to the summit; 1. c. 66 the lavas between 

 1 1 00 and 1420 m are classified as feldspar basalt. Anyhow, the fact that normal 

 olivine basalts form the base of the island helps us to understand why the topo- 

 graphy is so different from the configuration of Masatierra. 



Volcanic soils have a reputation for being rich in mineral nourishment; basalts 

 give rise to fertile soils. Quensel has published a number of rock analyses from 

 both islands to which I refer (1. c. 83 — 84). Of the samples of the humus layer 

 collected by us those from the lower levels of Masatierra were lost; only four 

 remain, two from each island. I wanted to get them analysed in the same way 

 as the rock samples in order to be able to compare them, but this met with in- 

 superable difficulties and 1 had to content myself with such dates as could be 

 supplied by the Bureau of Agricultural Chemistry in Stockholm (Experimentfaltet), 

 which I have to thank for the figures under I to 4 in Table XI\'. I have added 

 figures for 3 rock samples according to Quensel (II, \T and \TI). With these 

 the figures for the soil samples are not comparable; the methods of analysis 

 are dift'erent. 



The soils are acid to moderately acid and, in comparison with the mother 

 rock, poor in Ca, K and P. But these figures tell us nothing about the mineral 

 contents of the underlying clay; they refer to the humus layer. 



Soil tonperatures. The temperature of the soil was observed in 16 places on 

 Masatierra from near sea level to the Portezuelo pass. The apparatus used, con- 

 structed by the late Professor A. Hamberg, was a sharp-pointed iron-tube, into 

 which the thermometer was lowered. The wall of the tube was perforated on 

 level with the mercury, and the tube was closed with a stopper. The greatest 

 depth reached was 50 cm. Table X\*. 



The figures for stations 12 — 16 are high. The soil is poor, thin and exposed 

 to the sun, it dries quickh- and does not allow the seeds of forest species to 

 germinate. The new soil is taken possession of by the much less exacting introduced 

 species or, if exposed to erosion, will remain a desert. 



At loc. 4, where a stream crosses the trail, the temperature of the water was 

 taken on three occasions. Table XVI. 



The stream comes from the Portezuelo ridge and is considerably cooler than 

 the soil in the same altitude, c. 2;o m. 



