CI. W. Robinson 347 



by the seventh value from the top of the list. If the number had been 

 28, the quartiles would have been taken as midway between the 

 seventh and eighth values from the bottom and top respectively. In 

 the case in point the quartiles are 18-4 and 22-6 respectively. The 

 interquartile range is 22-6 — 18-4 = 4-2. In the table, the Umits are 

 given under the heading interquartile range and indicate the extremes 

 between which half of the results lie. Interquartile ranges are given 

 for the Anglesey medium loam and the Palaeozoic silt loam only, as in 

 the other cases not enough analyses are available to give any meaning 

 to this or any other measure of dispersion. One point remains to be 

 noticed. In a series of results lying between two limits, if the dis- 

 tribution is perfectly even, the interquartile range will be, ex hypothesi, 

 half the range. Where there is a tendency for the results to approxi- 

 mate to an average value the interquartile range will be less than half 

 the range. It will be seen on reference to the table that this is markedly 

 the case in those soil types for which interquartile ranges are given, the 

 interquartile range being generally about one-third of the range. 



1. Anglesey Medium Loam. 



This soil type occurs over the greater part of Anglesey. It is most 

 commonly the result of the weathering of schistose materials of Pre- 

 Cambrian age either in situ or in drift deposits. Soils formed from 

 Granite, Ordovician grits and Old Red Sandstone are so similar in 

 properties and composition to the soils formed from schists that they 

 are, for the present, classed along with them. 



The schists, from which the majority of these rocks are formed, are 

 an exceedingly interesting but complicated series of rocks. They are 

 of course metamorphic. The original materials from which they have 

 been formed are of two classes, namely igneous and sedimentary. It 

 has not been possible up to the present to correlate these rock types 

 with any distinctions in soil properties at present. Generally speaking 

 these Pre-Cambrian rocks are dark green, finely crystalline rocks, often 

 highly contorted and in many places containing quartz veins. 



In texture these soils are medium loams, often rather stony, of 

 considerable fertility. In colour {" ninvium ne crede colon") they vary 

 from chocolate to fairly light brown. The subsoils are much lighter in 

 colour and are often bright yellow. The depth is generally satisfactory 

 and the natural drainage is good in the case of sedentary soils. The 

 drift soils, generally in hollows, are not so well drained and, unless 



