628 BR. G. J. HINDE AND ME. HOWARD POX ON [Nov. 1 895, 



Even in the same quarry the radiolarian rock presents itself 

 under very different aspects. Some of the beds are of a very dark 

 or even black, intensely hard, brittle chert with a hackly fracture ; 

 others are of a nearly uniform dull grey tint or quite white, but 

 equally hard though less brittle, while yet other beds show alternate 

 light and dark bands which give a striped appearance recalling 

 that of the Carboniferous chert of Leyburn in Yorkshire, with 

 which this rock has been compared by Prof. Phillips. 1 Of a less 

 degree of hardness than those just mentioned are beds of dark to 

 light grey, siliceous, platy rock. In some of these the rock is mottled, 

 patches of dark and white alternate on the same surface-plane, and 

 verj T frequently the margins of the joint-planes are grey or white 

 while the interior of the rock is dark. 



Alternating with the beds of hard chert and platy siliceous rock 

 there are beds, mostly of a white or greyish tint and of a very soft 

 shaly character, which readily break up into thin flakes or laminae, 

 and in the softest kinds disintegrate when placed in water into a 

 very fine, cream-coloured mud. These soft incoherent beds are 

 filled with radiolaria equally as much as the hard chert, and, as 

 a rule, the organisms can be more easily recognized in the soft 

 than in the hard rock. The soft shales in some instances appear to 

 result from the partial decay of harder beds, for within them there 

 are in places thin layers of a bluish unchanged platy rock. The 

 shales seem to have been less affected by jointing than the harder 

 beds ; but, on the other hand, in some localities they show incipient 

 traces of cleavage by delicate parallel lines crossing obliquely the 

 planes of lamination. They are also much less common than 

 the harder rocks ; generally a single bed of not more than 1 or 2 

 inches in thickness intervenes in a considerably thicker series of 

 cherty or platy rock, and in some exposures the very soft shaly beds 

 are altogether absent. 



There are numerous gradations between the extremely hard and 

 compact cherty rocks and the very soft shaly beds ; not infrequently 

 a very thin lamina of the softer shale is interposed as a partition 

 between harder beds. The central portion of the beds of the cherty 

 rock is generally harder and denser than the exterior layers. 



In addition to the regular bedding the radiolarian rocks through- 

 out, both hard and soft, show that they are built up of a series of 

 thin laminae. These are seen best in some of the light-grey harder 

 rocks, as so many delicate parallel lines of slightly varying lighter 

 and darker tints which succeed each other with great regularity 

 and evenness throughout the beds, thus indicating extremely 

 slow and long-continued conditions of tranquil deposition and 

 consolidation. 



1 'Pal. Foss. Cornwall, etc.,' 1841, p. 190. 



