﻿220 DE. H. C. SORBY ON THE APPLICATION OF [May I908, 



In the case of pyrites in chalk, it seems clear that this pushed 

 aside the chalk and did not include it. On the contrary, when 

 concretions of carbonate of lime or iron were formed in sandstone, 

 they appear to have filled up the interspaces between the grains, 

 and did not displace them. In some rocks, both kinds of action may 

 have combined. 



XVI. Spots in Welsh Slates. 



Many years ago I made nearly 200 measurements of these spots, 

 determining the length of greatest and least axes to about a 

 hundredth of an inch, excluding those manifestly much influenced 

 by stratification. 



At that time I little thought what interesting conclusions could 

 be drawn from them, and my object was merely to ascertain the 

 amount of compression to which the rock had been subjected when 

 the cleavage was developed. I even then saw that the original spots 

 were not spheres, and endeavoured to make my observations so that 

 the stratification was inclined at all angles to the cleavage, and the 

 spots could be treated as spheres in making my calculations. In 

 some cases this was approximately correct, but not in others ; and mj'^ 

 results were not sufficiently accurate for the purpose now in hand. 

 I devoted therefore some weeks to the discussion of the observations, 

 my aim being to learn what was the condition of the material when 

 first deposited ; the period in its history when the spots were formed ; 

 their exact shape ; the extent of the consolidation to which the rock 

 had been exposed, before cleavage was developed ; the amount of 

 lateral compression caused by the elevation of the Welsh mountains ; 

 and the change in dimensions and structure of the rock, caused by 

 the compression. Since this may appear an impossible programme, 

 it will be necessary to enter into much detail, in order to show how 

 the facts enable us to learn all these particulars approximately. 



In many cases, when measuring the axes of the spots, I alsa 

 determined the direction of the stratification ; but this could not 

 always be done, and in some cases there is so much contortion that 

 the conclusions are doubtful. I invariably kept separate measure- 

 ments made in the plane of cleavage and perpendicular to it. They 

 were made in a quarry at Bethesda, in one at the bottom of Nant 

 Ffrancon, in the great Penrhyn quarry, and in sundry quarries near 

 Llanberis. The first step was to calculate the relative length of the 

 longest axis, that of the shortest being taken as unity. In some 

 places, where the stratification was fairly uniform, this ratio did not 

 differ materially in as many as 26 cases ; but in others there was a 

 wide difference, because of the stratification cutting the cleavage at 

 various angles. This was visible at once by marking the value of 

 each measurement along a straight line. It was then seen that, 

 for certain calculations, the measurements in the quarry at Bethesda 

 were extremely good, since the long axis of the spots seems to he 

 inclined at all angles to the cleavage. Those in the quarry at Penrhyn 

 in a plane perpendicular to the cleavage were also well distributed 



