DIASTROPHISM AND THE FORMATIVE PROCESSES 521 



such data as we have, if only to see what would be their meaning 

 if the facts were as they are guessed to be. Without any pretention 

 to accuracy, permit me to assume, on the basis of rough inspections, 

 that for the total post-Proterozoic outgrowth the landward addi- 

 tions to be made to the present shelf are of the order of 40 per cent 

 of its area, and that the subtractions are of the order of 20 per cent. 

 My real judgment inclines to make the former more nearly 50 per 

 cent and the latter more nearly 10 per cent; but, proceeding on 

 what seems to me a conservative assumption, the total area of out- 

 growth of the post-Proterozoic circum-continental terrace, neg- 

 lecting the crumpling, compacting, and shortening that have arisen 

 at times from lateral thrusts, amounts to 12,000,000 square miles. 



The coast line of the present continent is sometimes taken as 

 125,000 miles. For our present purposes a measurement on less 

 curved lines such as to represent the middle of the shelf belt en- 

 circling the continents is more suitable, and an estimate on this 

 basis gives about 100,000 miles. The mean breadth of the con- 

 structive terrace will then stand at 120 miles. Checked by direct 

 measurements in some of the best determined representative cases 

 this seems conservative. 



Let us now turn to the sources of sedimentary supply. It 

 would lead to large errors if each unit of the sea border were assumed 

 to be fed with sediment from the whole area of a strip of land 

 abutting on it and reaching back to the heart of the continent, for 

 in the first place, much of the drainage runs away from the coast, 

 and, in the second place, there is more or less coastwise drift that 

 denudes some tracts and builds out others disproportionately. 

 These factors have no doubt varied from age to age, and perhaps 

 varied greatly, but our immediate purpose is to secure a rough 

 concept of processes as they now are and as they are related to the 

 present accumulation of sediments. These may be qualified later 

 to fit the mean secular conditions or the specific conditions of any 

 particular problem. 



At present the continental surfaces may be divided into three 

 classes in respect to the transportation and lodgment of sediments: 

 (i) transportation into interior undrained basins which contribute 

 nothing immediately to circum-continental growth; (2) trans- 



