internal Structure of the Magnesian Limestone. 89 



scattered upon the crown of the escarpment on both sides of the village of Maltby, and might at 

 first sight be mistaken for enormous boulders ; but on examination thoy are found to be in situ 

 and are the hard indestructible remains of beds which were once continuous, and of which the 

 softer portions have been washed away. Striking appearances of the same kind may be seen on 

 the escarpment near North Anston, and in some other places in the course of its range through 

 the southern parts of Yorkshire. 



To this modification may perhaps be conveniently referred some of the varieties of magnesian 

 limestone which appear in the Gill quarries above Sunderland Bridge. In this locality the rock 

 when considered on a great scale, has not a concretionary structure so evidently exhibited as in 

 some of the preceding examples, and is more interrupted by vacuities containing earthy pulve- 

 rulent matter. The harder specimens, which are of a smoke-grey colour, answer however very 

 well to the description given above. 



The various masses described under this sixth modification of the magnesian limestone, appear 

 to have been regularly deposited with the other parts of the formation. But being composed of 

 an indefinite mixture of carbonate of lime and magnesia, with certain impurities which could not 

 entirely combine into a true dolomite, and being prevented by some internal cause from cohering 

 and forming a solid rock, the particles, after deposition, seem to have undergone great internal 

 movements, — to have run into lumps and masses more or less crystalline, rejecting great portions 

 of earthy residuum, — and in this way to have produced that complexity of structure, and those 

 cells and vacuities which are above described. 



This concretionary rock is never a true breccia ; and the causes which produced its structure 

 having acted irregularly, it is very seldom bounded by plane surfaces. Hence that false appear- 

 ance of want of conformity, presented by some of those sections, in which the amorphous masses 

 of yellow limestone are seen to overlie the stratified parts of the formation. (See Plate VII. 

 figs. 2. & 4.) 



VII. Beds, and irregular concretions of Crystalline Limestone, without 

 Magnesia. — Masses of dolomite of more or less perfectly crystalline texture 

 are, as before stated, found subordinate to the earthy beds of magnesian lime- 

 stone; butj in some rarer instances, beds, or more properly, irregular concre- 

 tions of carbonate of lime, seem to have separated themselves from the other 

 parts of the formation, at the time of its passing into a solid slate. Of these 

 we have some remarkable examples in a quarry about one mile south of Ripon, 

 where great masses of nearly pure limestone (of a grey, smoke-grey, or cloudy 

 bluish-grey colour, of a porous texture and crystalline structure) arc irre- 

 gularly imbedded among the soft earthy strata of this formation. These 

 masses cannot have been formed by infiltration ; and they are too much 

 blended with the other beds (passing insensibly into them, or penetrating 

 their substance in the form of strings or small contemporaneous veins), to be 

 regarded as mere accidents of deposition : they must, therefore, be considered 

 as further examples of that irregular concretionary structure which I have 

 been endeavouring to illustrate. (See Plate VII. fig. 3 ) 



It may be convenient to mention in this place some masses of crystalline 



VOL. III. SECOND SERIES. N 



