T. M. Hall — Extent of Geological Areas. 353 



formation. Proceeding on this reasoning, it occurred to me that, 

 instead of the very troublesome and uncertain method of bringing the 

 areas into figures and measuring their superficial extent, it might be 

 quite possible to arrive at equally good, if not better results by means 

 of scales and weights — in other words, by cutting out in the paper 

 the areas according to their outline, and then weighing them sepa- 

 rately, the products being taken either as parts of the whole, or 

 compared with one another. To insure success, in addition to 

 accurate weights, an absolute uniformity in the quality and weight 

 of the paper is a sine qua nan, and the separate sheets on which the 

 Ordnance Maps are printed would not always fulfil this latter re- 

 quirement. I was consequently led to adopt a safer and less ex- 

 pensive method than that of cutting up the maps themselves. The 

 area of Devonshire being so large, measuring on the scale of one inch 

 to the mile, six feet in height, by five feet eight inches in width, I 

 could not procure any single sheet of paper of these dimensions : 

 however, two or three copies of the " Times " afiorded an excellent 

 substitute, and as it is manufactured by rolls of some miles in length, 

 it may be supposed that it would be uniform both in quality and 

 weight. To prove this, I first laid the sheets over the Ordnance 

 Maps, and cut out very carefully the boundaries of the whole county. 

 Then taking the square mileage of Devonshire from the official 

 returns, I measured off with a rule as large a piece of paper as 

 would represent the square root of this known quantity. The weight 

 of the two sheets when put into the scale exactly coincided, viz. 

 107-5 grammes. Being satisfied thus far, I proceeded to cut out in 

 the paper all the formations according to the boundaries laid down 

 by the Geological Survey. These portions were weighed separately, 

 placing all the strips representing alluvium together, then all the 

 greensand, and so on down to the Carboniferous and Devonian. The 

 results were then calculated as proportional parts ; the whole area 

 of the county being taken as 100, and by this means I obtained the 

 following figures : — 



Metamorphic Rocks (Start Point and Bolt Head) ... 0-74 



Devonian series ... ... ... ... ... ... 25'71 



Carboniferous ... ... ... ... ... ... 41"93 



Granite 9-88 



Triassic series ... ... ... ... ... ... 14"44 



Lias 0-37 



Cretaceous 4-47 



Miocene ... ... ... ... ... ... ... 4'46 



Alluvium 2-00 



lOO'OO 

 I should add for the information of those who may wish to try the 

 experiment, that as newspaper is too thick to admit of tracing the 

 boundary lines through it, 1 laid a large plate of glass on the map, 

 and with a pen traced the lines on its surface. Before the ink had 

 time to dry, the paper was spread on the glass, when the lines were 

 transferred to it, as in a lithograph. 



DECADE II. — VOL. V, — NO. VIII. 23 



