OKIUIW ANl) DEVELOPMENT OF ORE-DEPOSITS, 181 



Usually in the granites and elvans of Cornwall the material 

 of the spheroids differs but little from the surrounding rock- 

 substance. In crystalline rocks the crystals may be a little 

 larger, — or occasionally a little smaller — the rock may be, and 

 usually is more compact in the spheroid, and there may be, and 

 often is, a greater proportion of some one mineral component 

 present. These two latter causes are probably the reason of the 

 curved" joint " which separates the spheroid from the matrix. 



The structure is so common in the eruptive rocks of the 

 West of England that scarcely any quarry or road-cutting can 

 be examined which does not afford examples more or less perfect. 

 The best examples are afforded by the mica-traps near Falmouth. 

 That which is seen near the Penryn saw mills, where the dyke 

 is only 3 or 4 ft. wide, is almost entirely composed of spheroids — 

 many of which shew, by weathering, three or four concentric 

 laminse, although in the fresh state the joints only appear when 

 the rock is struck with a hammer.* Some spheroids in the dyke 

 of this rock which appears in the cliffs below Mawnan Church, 

 are not less than 8 ft. in diameter. 



Spheroidal structure is often visible in the sandstones 

 and grits of the neighbourhood of Ladock — this too is accom- 

 panied by a concretionary accumulation of phosphate of lime, 

 but only to a very small extent- 



The structures known as spherulitie and perlitic are related 

 to the spheroidal structure, being curved joint structures — but 

 they do not appear to be in any way associated with concretions. 

 On the whole we may say that concretions are of little economic 

 importance in unstratified mineral deposits, and especially in 

 such as are most usually found in the West of England — though 

 they are often of vital importance in bedded deposits — as for 

 instance in the clay ironstones of the Coal Measures, the phos- 

 phatic deposits of the chalk, and the galena of the Mechernich 

 sandstones. 



Sporadic crystallization and crystal aggregates. Tn an incipient 

 state, sporadic crystallization shews itself in the spotted schists 

 which are so common on the borders of the various granitic 



* See R. ISr. Worth, Eep. M.A., 1883. See also Geol. Age of Cornwall and 

 W, Devon, Journ. RJ.C, 1884, p. 39, 



