372 ARRAN. GEOLOGY. SECONDARY STRATA. 



to know, and give a general account only of the most 

 remarkable strata. 



Among these, the predominant are red sandstone, red 

 conglomerate, white quartz sandstone, calcareous grit, 

 blue limestone, and shales or schistose marles of various 

 colours, brown, black, red, purple, mottled, and grey. 

 The red conglomerate and the sandstone, here as else- 

 where, occasionally contain calcareous matter as well as 

 mica. The red sandstone varies in colour from a 

 .brown red to a blueish purple, and is characterized 

 by globular or irregular stains of white, often disposed 

 in a very ornamental manner, as well as by white veins. 

 No adequate explanation has, as far as I know, been 

 offered of this very singular but well known appearance; 

 from which this variety has, among some observers, ac- 

 quired the name of variegated sandstone. That it does 

 not possess those constant geological relations which have 

 been assigned to it, will be readily deduced from the history 

 of this series. Besides this, some of the beds which 

 present an uniformly red colour, are found to possess a 

 globular concretionary structure. That structure is how- 

 ever invisible on a fresh fracture, and is only discovered 

 by the action of the sea, which, exerting an unequal power 

 on its different parts in proportion to their relative hard- 

 ness, leaves in many cases a botryoidal surface. Chemical 

 examination detects no difference of composition between 

 the globules and the intermediate parts. I may here 

 add, that other modifications of internal structure are 

 discovered in a similar manner. The most remarkable of 

 these varieties is cancellated ; the rock wasting until it 

 becomes a honeycombed mass, and the caverns being irre- 

 gular and separated by thin lamellae. Analogous to this 

 is the appearance of thin and reticulating veins, found 

 protruding on the surfaces that are exposed to the sea; 

 these being the edges of entangled and crossing laminae 

 which possess a hardness superior to that of the general 

 rock. 



