internal Structure of the Magnesian Limestone. 87 



concretions; and when the laminte disappear, pass into slaty beds, and 

 sometimes into thick smoke-g-rey fetid beds of nearly compact structure. 



Of these laminte, some are shining and crystalline, others are dull and earthv. 

 The very thin laminae of the latter variety, which occur in abundance near 

 Marsden Rocks, are often slightly flexible ; and very fine specimens of flexible 

 magnesian limestone with thicker laminae, occur in a bed near the middle of 

 the cliff. 



V. Earthy Magnesian Limestone. — This modification does not occur in 

 any striking form to the south of Doncaster ; but, in the range of the forma- 

 tion from that place through Yorkshire and Durham, it exists in the greatest 

 abundance. It is in some places hard, coherent, and regularly bedded ; in 

 other places it becomes soft as chalk, and stains the fingers ; occasionally it 

 loses all the marks of stratification, and passes into great pulverulent masses 

 only held together by the veins and harder concretions which pass irregularly 

 through them*. It will, however, be better to describe this last variety as 

 subordinate to other modifications of the formation, especially those which 

 exhibit a great or small concretionary structure. 



So far I have endeavoured to describe those modifications of the formation 

 which make an approach to a simple structure : but whatever may be their 

 mineralogical or economical importance, they are developed upon a much less 

 scale than some of the varieties I am now about to notice. 



VI. Large irregular Concretionary Structure. — Under this subdivision are 

 included all those parts of the deposit which are composed of great irregular 

 concretions, and in which the subordinate parts exhibit a complex irregular 

 structure. These modifications may often be traced in the separate beds of 

 an escarpment which is distinctly stratified. They are, however, seen in the 

 most impressive form, when, to aU appearance, an entire system of beds has 

 been so changed, that the lines of deposit are obliterated, and the whole 

 escarpment shows an amorphous mass of crystalline, compact, cellular, and 

 earthy materials rudely blended together, and apparently passing into each 

 other without order or arrangement. Phaenomena like these do not admit of 



* The proportion of lime and magnesia in such specimens as these is found to be extremely 

 variable. Two pulverulent varieties (one found near llipon, and the other near Kiiaresborough,) 

 proved, however, to be nearly identical in composition. The following analysis is by Mr. Holme. 



2. Earthy variety from Knaresborougk. 



1. Earthy variety from Ripon. 



Carbonate of lime 71.125 



Carbonate of magnesia 25.625 



Red oxyd of iron and alumina .... 1.750 



Silica a trace of, and water 1.500 



100 grs. 



Carbonate of lime 72.000 



Carbonate of magnesia 25.500 



Rod oxyd of iron and alumina 1.000 



Silica a trace of, and water 1.500 



100 ^rs. 



