J. Love—Comparative Fertility of Soils. 133 
formations within its boundary, and it seems fair to presume that 
these may have a considerable influence in determining the distribution 
and variation of its flora. Of the 1,310 species of plants figured and 
described in Bentham’s British Flora, about 1,000, or approximately 
three-quarters of the whole flora, have been found growing in Surrey ; 
these are arranged with the rocks over which they grow in the 
following order “of diminishing numbers :— 
Rock ForMATIONS. SPECIES OF PLANTS, 
Lower Greensand... Bae She ere ae 711 
Gravel ... Ace ie oe Ade are 655 
Chalk we oe aie ie rad ae 523 
Bagshot Beds... ane BE bis ds 513 
London Clay ae sas sat 26 aD 496 
Weald Clay ys 34 a BAe 364 
Gault and aes Greensand... on oe 356 
Alluvium ... aes on a: ae 302 
Hastings Beds. Se aa ang 281 
Woolw ich and Reading Beds Bi ae Be 214 
Thanet Beds we 204 
Perhaps it might be supposed. that these ‘numbers of plants would 
accord with the extent of area of the rocks over which they are 
growing. ‘To determine this point the areas of the rocks which form 
the surface of the land within the county boundary of Surrey have 
been measured, and are arranged in the following order of diminishing 
areas, the figures giving the number of square miles of each 
formation :— 
Rock Formations. AREAS. 
Weald Clay one vee 500 ae se 165-4 sq. miles 
Lower Greensand Ses a eee aoe 149 Gees 
Chalk... be ae ike aac fe UIP) 4 
London Clay Ae Re ine ly BA USP as 
Bagshot Beds... aoe ae see sae 93°84 ,, 
Gravel a AG me ne Hoe Garo4ueee 
Woolwich and Reading ee ci Ay, ‘isis 23°84 ,, 
Alluvium .. er i558 Sis Dele A 
Gault and Upper Greensand... ae oe OR en 
Hastings Beds... ae Aa ae iW ee es 
Thanet. Beds ane ne ‘ Ae 4°33, 
Dividing the number of species of plants growing over each 
formation “by the number of square miles of the exposed surface 
of that formation will give the number of species of plants per square 
mile of rock surface. This may be called the density of species. 
They are the following, again in the order of diminishing numbers :— 
SPECIES PER 
Rock FormaTIons. Square MILE. 
Thanet Beds... tee ne aye ae 47-1 
Hastings Beds ... oc ane sfa 24°65 
Gault and epee Greensand |. ae cae 22°65 
Alluyium.. o. Ms is 13°66 
Gravel ... a se aS Ret 10°26 
Woolwich and Reading .. ia see A, 9-0 
Bagshot Beds... ioe +e 5 be 5-47 
Lower Greensand ie . Sate nae 4°75 
Chalk - .:. on cae Be Ass Bae 4°39 
London Clay ... - ... Boa Ge ae 4°38 
Weald Clay Ane ae me he nec 2°21 
