262 DB. A. GILLIGAN ON THE PETEOQBAPHY OE [vol. 1XXV, 



while the lowest is about O075. It is of interest and importance 

 to note that liquid cavities with movable bubbles occur in the 

 quartz of the granite, gneiss, and quartz-porphyry pebbles described 

 in an earlier part of this paper. 



(2) Felspar. 



The grains of felspar are commonly well rounded, as is usually 

 the case in sedimentary rocks, and often somewhat elongated, this 

 form having been determined by cleavage. As with the larger 

 pebbles, the dominant felspar in the finer material is microcline, 

 next in order comes microcline-microperthite, then oligoclase, and 

 orthoclase. Very basic felspars appear to be altogether absent in 

 the fresh state, and were never, I believe, present in any notable 

 quantity. These felspars are often exceedingly fresh. As in the 

 larger pebbles, the cross-hatching of the microcline is wonderfully 

 well shown, while the oligoclase is frequently quite unaltered 

 (PI. XVI, fig. 1). _ 



Microcline is, as is well known, the most resistant of the felspars 

 to alteration, and is found quite fresh in deposits of all ages. The 

 oligoclase and orthoclase are, however, much more readily decom- 

 posed, and their freshness here has a deep significance, which will 

 be emphasized and discussed when the mode of derivation and 

 accumulation of the material comes under consideration. 



It appears probable, from the frequent association of calcite with 

 decomposed felspar, that the original form of the felspar had been 

 one of the less basic soda-lime group. This was especially well 

 seen by me in some sections of the grit from a borehole at Mean- 

 wood, Leeds. 



An investigation into the question of the original freshness of 

 the felspar and its alteration by subsequent atmospheric action 

 was made by means of sections cut of the grit from a deep boring 

 at Bradford, and it was found thai, as a general statement, it may 

 be asserted that the percentage of fresh felspar increased with the 

 depth from which it was obtained. In this connexion a further 

 confirmation is to hand in the fact that it sometimes happens 

 that a borehole put down in the grit for water fails to tap the 

 supply, since the density of the rock approaches that of a true 

 granite, and hence its porosity is very low. The rock has to be 

 blasted at the bottom of the borehole, so as to open up a con- 

 nexion with a joint through which the water may find its way to 

 the borehole. 



Inclusions in the Felspars. 



Quartz-blebs are very common in the microcline, while zircon, 

 tourmaline, and rutile also occur, but of these zircon is by far the 

 commonest. Muscovite, as a primary inclusion and not due to 

 decomposition, is also fairly common. 



