FROM THE CROSS FELL INLIER. 521 



distinguish tlio felspar from the quartz; tlio great majority of the 

 grains are simple, but here and there one shows twin-striation. It 

 will be noticed below that the diabase uear this locality gives evi- 

 dence of great crushing. 



The large igneous mass on Thack Moor and the intrusions in 

 Ousby Dale, apparently offshoots of it, belong to the same general 

 typo as the Dufton intrusive rock, but lack white mica. In 

 all these rocks the closeness. of the porphyritic crystals, obscuring 

 the groundmass, gives a very crystalline appearance in the field or 

 in hand-specimens. 



Certain intrusive rocks, such as the large mass to the south of 

 Cocklock Scar and some dykes, e. (j. at Maiden Way in Ardale Beck, 

 show a considerable departure from the foregoing and an approach 

 to the characters of the laraprophyres described below. In hand- 

 specimens they have a less crystalline appearance than the quartz- 

 porphyries, and quartz is not recognized. The general aspect is 

 that of some so-called porphyrites, the colour being brown with 

 little stained felspar-crystals and facets of dark mica and augite. 

 The slices [916, J)22] bear out the idea of a transition to the 1am- 

 prophyre type. Besides the greater abundance of dark mica, mostly 

 undergoing a greenish alteration, we notice the coming in of abun- 

 dant apatite and magnetite, and especially of augite in perfectly 

 formed crystals, now completely pseudomorphed. The groundmass 

 of these rocks is much decomposed, and it is difficult to decide 

 whether any part of the free silica is original. 



5. The LAiiPROPHrREs. 



It would not be easy, and perhaps not very profitable, to attempt 

 any such division of the " mica-traps " of the Xorth of England as 

 that between minettes and kersantites. A distinction founded on 

 the crystallographic systems of the felspars is more futile than 

 usual in this case, since those minerals are usually too far destroyed 

 for recognition. Analysis might, of course, show the relative pro- 

 portions of potash and soda present ; but as this does not seem to 

 bear any relation to the percentage of silica, &c.*, the character of 

 the original felspar in any case would import little as regards the 

 essential nature of the rock. The family-name " lamprophyre " is 

 therefore sufficiently precise. In some of the rocks, as in Eosen- 

 busch's " vosgesite," idiomorphic augite accompanies or to some 

 extent takes the place of mica. 



Like most lamprophyres, these rocks are very prone to decomposi- 

 tion by weathering agents, and are often impregnated with secon- 

 dary carbonates. The freshest examples have a dark grey ground 

 jjlentifully spangled with flakes of dark brown or nearly black mica, 

 sometimes as much as an inch in diameter, but usually much less. 

 In more altered specimens the mica takes a deep brown colour, with 



* See Bonney and Houghton, Quart. Journ. Gaol. Soc. vol. xxxv. (1870) 

 p. 165. 



