466 Prof. Sedgwick on the Structure of large Mineral Masses. 



of essential importance (viz. the passage of the laminations of deposit through 

 the globular travertine), I may be pardoned for considering it of no value to 

 the argument. Under such circumstances, however, it may be proper shortly 

 to ffo over some of the grounds that led me to the conclusion above stated. 



1st. The niagnesian limestone is sometimes stratified in very thin earthy 

 beds or laminae ; and occasionally (e. g. on the coast of Durham, a few miles 

 north of Hartlepool,) organic remains may be traced in abundance along the 

 lines of such strata, which must, therefore, have been formed by a slow de- 

 position of the successive layers. 



2ndly. Globes of various sizes are found in such beds, sometimes appear- 

 ing as mere semi-indurated lumps, sometimes as hard concentric concretions, 

 sometimes as balls of a radiating crystalline texture : and through such masses 

 (except in the ultimate state of perfect crystallization) we can often trace the 

 lamina of stratification passing uninterruptedly. This fact alone is decisive, 

 and can be established even by hand specimens. 



Srdly. In other places the same limestone is found in thick beds, and is used 

 for building. Such beds are not, however, a momentary product; because we 

 can often subdivide them into smaller beds, and even into laminations. Now 

 we may sometimes discover (especially on producing a fracture with a pon- 

 derous hammer,) that portions of such beds are entirely made up of spherical 

 concretions, interfering with each other, and producing solids with trapezoidal 

 faces. These concretions must be contemporaneous ; but their centres are 

 found in beds which are not contemporaneous : therefore the concretions 

 must be posterior to the stratified masses in which they are found. This 

 seems to me nothing short of demonstration. 



4thly. The formation, though stratified, is often irregularly solidified: por- 

 tions are solid ; other portions are perfectly earthy and pulverulent. Now, 

 we may observe two things : first, the laminae, when they abut against the 

 earthy portions, are often crystalline ; secondly, there are often packed up in 

 the incoherent magnesian earth large spherical concretions of nearly pure 

 carbonate of lime, which do not adhere to or touch any of the solid portions 

 of the surrounding strata. How is such a structure to be explained except by 

 a slow chemical segregation of parts ? 



I know that travertines sometimes exhibit spherical aggregations. Indeed 

 1 saw many magniflcent examples of such formations when, in 1829, I visited, 

 along with Mr. Murchison, some deposits of travertine in Bavaria. But the 

 spheroidal portions we saw were fixed to the solid mass, and had no laminse 

 of deposit passing through them. Whenever such laminations are present, 

 (whatever be the age of the formation,) we may, I think, be certain that 



