416 J. CLIFTON WAED ON THE MICEOSCOPIC STEUCTTJEE OF 



4. Some of the Cumberland lavas, in proportion as they lose the 

 acicular crystalline structure, acquire a likeness in some degree to 

 the Welsh felstones, in the general character of the base. 



5. In lithological structure, the Cumberland lavas have, for the 

 most part, much more of a felsitic than a basaltic appearance. 



6. In petrological structure they have much the general character 

 of the modern Yesuvian lavas, the separate floivs being usually of no 

 great thickness, being slaggy, vesicular or brecciated at top and bot- 

 tom, and having often a considerable range, as if they had flowed 

 in some cases for several miles from their point of eruption*. 



In addition to these varied points of likeness and unlikeness, 

 this further fact should be considered, that the Cumberland rocks 

 are on the whole very different in microscopic structure and appear- 

 ance from such old basalts as those of S. Staffordshire and some of 

 those of Carboniferous age in Scotland. The reason of this may 

 be, that, their age being so great, their original structure has in 

 great part been obliterated. 



In what group, then, are we to place these old Cumbrian lavas ? 

 In external appearance they are felsitic, though somewhat different 

 in many cases from the Welsh felstones. In internal structure 

 they have considerable analogy with the basalts, though one would 

 often rather hesitate to call them true basalts or dolerites. In 

 chemical composition they are neither true basalts nor true felstones. 

 Certainly they seem to hold an intermediate position, somewhat 

 similar to that of the trachy-dolerite group or greystones of Scrope. 

 In composition they agree with porphyrite and melaphyre, as may be 

 seen by glancing at the table of rock-analyses (p. 417). Porphyrite, 

 however, if we accept that of Ilfield (Streng's melaphyre-porphyry) 

 as a sample, has a different microscopic character from the Cumber- 

 land lavas ; for Rose found that " thin polished plates showed a 

 transparent matrix marked with black spots and streaks, and filled 

 with black grains of irregular shape " f . Cotta, however, concludes 

 (p. 166): — " According to the known analyses, both microscopic 

 and chemical, the matrix most probably consists of an intimately 

 blended crystalline compound of oligoclase with augite or horn- 

 blende, magnetic iron-ore, and some apatite ; the fine acicular crys- 

 tals appear to be augite transformed into schillerspar ; " this seems 

 somewhat more nearly to apply to the rocks in question. Zirkel's 

 description of porphyrite, however, seems quite inapplicable $. 



The term melaphyre has been applied to so many different rocks 

 that it can only be used in a general way as denoting a rock of un- 

 certain place ; and in that sense only can the lavas in question be 

 Called melaphyres. 



* Details of volcanic centres in the district, and of the old volcanic 

 phenomena in general, will be found in the Survey Memoir on 101 S.B., now 

 in the press. 



f Rocks Classsifled, p. 170. 



| Mikroskopische Beschaffenheit, p. 404. 



