10 



J. A. PHILLIPS OX THE CONSTITUTION AND 



of angular pieces of quartz and felspar, of which some of the 

 larger fragments have a diameter of ^ inch ; they are cemented by 

 a siliceous concrete enclosing particles of granular quartz, through 

 which minute greenish microliths are plentifully disseminated. 

 Comparatively few fluid-cavities, either with or without bubbles, 

 are present. In addition this rock contains a few crystals of schorl 

 enclosed in the quartz, some water-worn flakes of silvery-white 

 mica, a few crystals of pyrites, and perhaps a little altered mag- 

 netite. The felspar chieliy belongs to a triclinic species, but ortho- 

 clase is also present. The rock at Dairy differs from that at Tregian 

 only inasmuch as it contains a few water-worn crystals of horn- 

 blende and a little magnetite. 



In addition to the foregoing, through the kindness of Mr. J. H. 

 Collins, I have been enabled to examine four other specimens of 

 Cornish grit, namely, one from St. Allen, four miles north of Truro, 

 two from Ladock, five miles further east, and one from Perranzabuloe, 

 on the Bristol Channel. 



Hand specimens of all these rocks closely resemble one another, 

 excepting that those from Ladock enclose numerous angular frag- 

 ments of a greenish slate, which the others do not, and that one of 

 them contains a number of rounded quartzose and other grains | inch 

 in diameter. 



When examined under the microscope, the St. Allen grit differs 

 little from those at St. Ewe ; the quartz is angular and transparent, 

 the largest fragments having a diameter of about inch, and the} r 

 sometimes, though rarely, enclose minute crystals of tourmaline. A 

 little hornblende, with white mica and epidote, are also present. 

 The felspar is, to a large extent, triclinic, but there is also some 

 altered orthoclase ; the quartz contains but few fluid-cavities. 



In the rock from Ladock, which contains small rounded grains of 

 quartz, felspar, and other material, these bodies are sparsely dissemi- 

 nated throughout the mass of the normal grit ; and a microscopical 

 examination shows that some of them are fragments of volcanic 

 rocks closely akin to the " greenstones " and " dunstones " of many 

 parts of Cornwall, but which have often become so altered as to be 

 recognizable only by their felspars and general structure. 



Pig. 2, PI. I., represents, in black and white, a fragment of volcanic 

 rock which occurs in this grit, as seen in polarized light, magnified 

 18 diameters. 



The second specimen from this district is made up chiefly of angular 

 fragments of quartz and felspar, united by the usual cementing con- 

 crete. It contains a considerable amount of felspar, a large propor- 

 tion of which is triclinic. The largest pieces of quartz are about 

 y L- inch in diameter ; and fluid-cavities, although by no means abun- 

 dant, are more plentiful than they are in the Cornish grits before 

 described. Some of the quartz contains a notable quantity of dis- 

 seminated epidote (?), and flakes of white mica are frequently jammed 

 between the fragments of which the rock is composed. A few minute 

 garnets are present, as well as some fragments of a volcanic rock. 



The microscopical structure of the gritty rock from Perranzabuloe so 

 closely resembles that of those from the other localities that a special 



