and the Distortion of Fossils, 417 



This amount of compression is greater" than I found for any- 

 other slates, excepting the remarkable slates of Tintagel and Garth. 



Example 5. — Green Grits of Llyn Padarn, Llanberis, Lower 

 Silurians lying below the Trap Beds of Snowdon. 



In this case I was enabled to obtain measurements of distorted 

 fossils in three distinct planes, and thus secure a kind of expe- 

 rimentum cruets for the mechanical theory of cleavage. The fossil 

 I made use of is Orthis expansa, of which the following are the 

 normal measurements : — 



Orthis expansa . Breadth of hinge . . . . = 238 

 > ■; Breadth of shell .... =289 



Length of shell =274 



Breadth of vascular impression =143 

 Length of vascular impression = 144 



By these measurements and those of the distorted fossils, I 

 obtained the following : — 



^ =33°, i= 1-433 



9 



i^'=43, ~ = l-552 



^iii lo .M;:.£;I^*u>^ <^" = 90°, -i^ = 1-961. 



r 



The third of these measurements gives a direct value of — , viz. 



1*961, with which the value calculated from the first two data 

 should agree. The agreement is very good, as appears from the 

 following : — 



a 



J = l-195 ^ (E) 



i = 1-881 

 c 



It is remarkable that the compression in this case, perpendi- 

 cular to the cleavage planes, is very small compared with the 

 compressions determined in the slates already discussed. This 

 doubtless arises from the fact, that sandstones resist the cleavage 

 action or pressure better than slate or mud. This singular fact, 

 of a less compression existing in sandstones than in slates, and 



accurate knowledge of the forms of the palaeozoic fossils was of the greatest 

 service to me, and he allowed me free access to all the specimens in the 

 Museum, and assisted my investigation greatly by his judgement. 



