ORIGIN AND DEVELOPMENT OP ORE DEPOSITS. 123 



Sometimes however, slate, otherwise good, is traversed by 

 millions of almost invisible joints, dividing it into prisms or 

 lozenges of less than a square inch area. Of course such slate 

 is absolutely valueless, but it is in precisely such circumstances 

 that the jointing becomes the most remarkable for its regularity. 

 In one of the quarries near Tintagel the writer found millions of 

 fragments of slate in the form of slender four-sided prisms 

 (pencil-slate). Each surface was smooth and flat, and the 

 utmost variation of the corresponding angles of inclination, as 

 measured by a contact goniometer, was less than 2 degrees. 

 Some of the needles were two inches long and less than an eighth 

 of an inch thick. Similar phenomena may be observed in shales 

 of all kinds in hundreds of localities around our coasts. 



Even columnar jointing, so characteristic of basalt and so 

 noticeable in the granite cliffs near the Land's End, is not 

 absolutely unknown in metamorphic and stratified rocks. Some 

 thick-bedded sandstones are pretty regularly divided into 

 columns, as might have been seen a few years since (perhaps 

 may now be seen) in the coarse sandstones near Bissick in Ladock 

 parish. A similar structure has also been noticed in the 

 Devonian limestones of Plymouth and Newton Abbot, but only 

 very locally. 



However the joints have been produced, it is quite certain 

 that the rending force has often been very considerable, since 

 we find in conglomerates pebbles of hard quartz or hornblende 

 rock torn across. This is common in the Ladock sand-stones, 

 and also in the conglomerates on the south side of the Helford 

 River. By the use of the microscope, this rending of particles 

 in the neighbourhood of joints is almost always to be detected, 

 unless the rocks are composed of very fine material, or unless 

 disintegration from chemical action is evident. 



See. 2. — Lamination and cleavage. These structures, so far as 

 they relate to rock-masses, may be generally defined as a tendency 

 to split more readily in certain directions than in others. They 

 may be traced to four causes, as under : — 



(1). The original laminar arrangement of the materials of 

 which the rock is composed. This is commonly called lamination, 

 and it is rarely found except in sedimentary rocks. The lamin- 



