406 J. CLIFTON WAKI) ON THE MICROSCOPIC STRUCTURE OF 



Three specimens of lavas, from widely separated parts of the dis- 

 trict, shall now be described in general terms, as representative 

 samples of a considerable number of specimens which have been ex- 

 amined and which will be treated of fully elsewhere. 



a. Cumberland Lavas, 



The contemporaneous traps of the Borrowdale volcanic series may 

 with as much reason be called lavas as any of the modern flows of 

 Vesuvius. The thickness of the separate flows is in general not 

 great ; their upper and lower surfaces are very usually slaggy and 

 scoriaceous ; and in many cases the vesicles, where they occur, are 

 drawn out in the direction of flow. I have seen some of these old 

 traps presenting an outward appearance almost exactly similar to 

 that of many a bed of lava round Vesuvius. Their thickness is, as 

 one would expect, subject to very sudden and wide variations; yet 

 many beds may be traced along their present outcrop for a distance 

 of several miles. 



1. Eycott Hill (Porphyritic Dolerite, Plate XVII. fig. 6).— This rock 

 is one of a thick series of lava-beds with vesicular portions, occurring 

 separated from the main mass of the Borrowdale Series at the north- 

 east corner of the district. Its lithological structure is that of a 

 compact base containing dark-green or black spots of a soft mineral, 

 and large felspar crystals, many of them an inch long. 



In fig. 6 the general microscopic structure of this rock is seen, with 

 the exception of the large felspar crystals. The base consists of a net- 

 work of acicular felspar prisms, the intervals being filled up with a 

 dirty green and brown pseudomorphic mineral, and numerous crystals 

 of magnetite. The large felspar crystals are triclinic, contain glass- 

 cavities and grains of magnetite, and are much cracked. Augite, in 

 crystals and grains, is interspersed with the other minerals ; much of 

 it is in the form of pseudomorphs {the soft dark spots before spoken 

 of); but there are some unaltered twin crystals of considerable size. 

 A portion of one of these is shown in the figure ; and in polarized light 

 the parts on either side of the faint median horizontal line show dif- 

 ferent colours, thus revealing the twin structure. The same crystal 

 contains several glass-cavities, in one of which are included two small 

 magnetite crystals and a bubble (see figure, close to a crack). Some 

 small parts of the crystal are replaced by a green mineral snowing a 

 faint dichroism, and which is probably chlorite. The many green 

 pseudomorphs scattered about are also slightly dichroic, as well as 

 parts of the dirty-green base ; it is therefore probable that in great 

 part the pseudomorphic mineral is chlorite or one of its allies, which 

 may be partly altered, forming the browner part of the base *: Some 

 of the green pseudomorphs have the appearance of being after 

 olivine ; and as that mineral has been found in some of these rocks, 

 its occurrence here is not at all unlikely. 



* This rock, in so far as it resembles an altered dolerite, might be called a 

 diabase. With regard to the greenish mineral, Zirkel remarks, under the head 

 of Diabase (Mikroskopische BeschafFenheit, p. 407), that it seems to be chlorite 

 and probably a decomposition-product of augite. 



