Vol. 50.] OF CARROCK FELL. 333 



must be regarded as the true line of junction. These scattered 

 prisms of plagioclase are embedded in a perfectly clear mosaic 

 of moderately coarse texture, which seems to be in some 

 cases of quartz, in others of felspar, mostly untwinned. The 

 structure is thus that which G. H. Williams x has termed ' micro- 

 poikilitic' The general appearance is as if the felspars had been 

 set free, by what was originally the isotropic base of the lava 

 becoming dissolved and absorbed into the gabbro-magma. The 

 material thus taken up is doubtless represented in part by the brown 

 mica which we find in the neighbouring gabbro, but the quartz 

 and the (probably acid) felspar of the clear mosaic are perhaps 

 to be referred to the same source. Mica is developed only excep- 

 tionally in the metamorphosed lava, and has somewhat different 

 characters from that in the gabbro. Its pleochroism is from a rich 

 brown to colourless, and there are intensely pleochroic haloes around 

 certain inclusions too minute for identification [1549]. 



Certain narrow veins, conspicuous in hand-specimens, pass from 

 the gabbro-j unction into the lava, and contain especially idiomorphic 

 brown hornblende moulded by quartz [1626]. Again the micro- 

 poikilitic areas may take on the form of little veins extending into 

 the lava [1553], or these may anastomose and spread for a short 

 distance from the junction. Indeed, the groundmass of the lava 

 very near to the gabbro seems locally to be replaced by patches of 

 clear quartz, etc., wedged in among the porphyritic felspars, the 

 needles of apatite, and the pyroxenes or their representatives [1625, 

 etc.]. Both pyroxene and magnetite seem in some places to have 

 been absorbed. 



It must be concluded from these phenomena that the gabbro- 

 magma has to some extent corroded away and incorporated in itself 

 the glassy base of the lava, and even in places some of its minerals, 

 in the immediate neighbourhood of the junction. Nevertheless all 

 the facts go to negative the idea that the lava has been to any 

 important extent melted down by the gabbro-magma. The felspars 

 have not apparently been dissolved at all. Even the little prisms 

 remarked as occurring outside the line of junction show no rounding 

 or diminution in size, and since they do not occur to any 

 greater distance than a fraction of an inch, it seems that very little 

 of the rock can have been removed. The blocks of lava, and the 

 fragments into which they are divided by veins of gabbro, are 

 sharply angular. 



The same inference might be drawn from an examination of the 

 gabbro near its junction with the lavas. The rock here, and for a 

 few feet, always contains brown mica, a mineral foreign to the 

 normal gabbro. The mica, indeed, occurs nowhere else in the mass, 

 with an exception to be noted below. At one or two spots where 

 the mineral was noticed no lava was actually exposed, but these 

 places were on the line of strike of the Eycott group as seen not far 

 away, and doubtless mark the position of concealed patches of lava. 



1 Journ. of Geol. vol. i. (1893) p. 176. 

 Q. J. G. S. No. 199. 2 a 



