﻿Yol. 64.] QUAiq-TITATIVE METHODS TO THE STUDY OF EOCKS. 217 



material in which the concretion was formed, and on the amount 

 of contraction which had previously taken place. If the original 

 deposit consisted of particles more or less completely symmetrical 

 in all directions, a considerable amount of contraction would produce 

 little or no effect, and the growth of the concretion would take place 

 equally in ail directions. This would explain why the concretions 

 in the Magnesian Limestone are almost perfect spheres. In 

 the case of those in line-grained sandstone at Arborthorne (near 

 Sheffield), the axis perpendicular to bedding is 90 per cent, of those 

 in its plane. The thickness of sand quickly deposited is reduced to 

 89 per cent, when well shaken, or to about the relative length of 

 the shorter axis in the Arborthorne concretions, and is exactly the 

 same as the mean for them and the green spots in the Old Red 

 Sandstone. The shortest axis in one from shale at Woodbourne is 

 49 per cent, of the mean of those in the plane of symmetry. When 

 line clay which has subsided for a day is allowed to subside for a year, 

 the contraction is 54 per cent, of the original, and would have been 

 close on 49, if the measurement had been taken earlier in the day, 

 or if the clay had been subjected to the pressure of superincumbent 

 deposit. The explanation that I would suggest is that, as may be 

 seen with the microscope, many of the fine particles of clay are flat 

 Hakes, and these to a great extent subsiding as compound granules 

 would originally be inclined at all angles to the horizon. Then, 

 when the deposit contracted vertically, they would become less and 

 less inclined, and the permeability for water would be increased 

 in the plane of symmetry and decreased in a line perpendicular 

 to it. In such a case, the relative length of the short axes of the 

 concretion would indicate the approximate amount of contraction. 

 This conclusion would, however, apply to a particular class of rock 

 alone, and then only approximately. 



Special Examples. 



The concretions in the Magnesian Limestone seen along the 

 coast north of Sunderland vary much in size, but when not 

 interfered with by others are almost true spheres, the axes being 

 100 : 100 : 100, and when they leave impressions upon one another 

 these are almost perfect circles. In my best specimens no axis 

 differs from the mean by more than a hundredth of an inch. The 

 nuclei must, therefore, have been small, and the surrounding 

 material of very uniform structure in all directions. 



The concretions in a fine-grained sandstone at Arborthorne (near 

 Sheffield) are a good example of those which have the axes in the 

 plane of stratification nearly eqnal, and that perpendicular to it 

 shorter. The means for the bigger specimens are 92 : 100 : 100, and 

 for the smaller 87: 100 : 103 — thus showing the greater effect of the 

 nucleus on the smaller, and in both cases the effect of the different 

 structure of the rock in a direction perpendicular to the bedding. 



A red sandstone used in Gloucester, which looks very like some of 

 the Old Red Sandstone of the Black Mountains, contains very perfect 

 green spots, varying in size from very small up to 1 inch or more in 



