(500 ) 
or, noticing that 
e, , mat a 
Xn Ly (a) sin Ses I, (a); 
1.3 ra pe 
we have 
7 
I, (a) = cosa +f a sin (a — B) dp. 
t 
0 
If we differentiate this equation, we find 
a 
I, (a) = sina (8 cos (a— 3) dp 
0 
from which we conclude that 
TB), pe 
F sin BdB = 1 — cosal, (a) — sin a 1, (a), 
0 
UP 
eo cos 8 dp = sin a I, (a) — cosa I, (a). 
J | 
Geology. — ‘Contributions to the knowledge of the sedimentary 
boulders in the Netherlands. 1. The Hondsrug in the province 
of Groningen. 2. Upper Silurian bouiders. — First commu- 
nication: Boulders of the age of the Eastern Baltic zone G.” 
By Dr. H. G. Jonker. (Communicated by Prof. K. Marry). 
This communication introduces the description of the Upper Silurian 
boulders of Groningen and its surroundings, in which my contribution 
that treats of the Cambrian and Lower Silurian erraties and appeared 
in 1904, is continued (36). The circumstance that in the summer 
of last year I had an opportunity of getting more intimately acquainted 
with the Seandinavian-Baltie strata by investigations of my own 
has aided me considerably in the study of these younger rocks. 
Owing to nearly a month’s stay in Gothland I managed to collect 
a great number of different species of rocks together with fossils 
characteristic to them in order to compare them with erratics that 
are found here. Much I owe to the kindness and assistance of 
Drs. O. W. WeNNERSTEN, who accompanied me on some excursions 
and whom I had very often reason to admire for his extensive 
knowledge of his native country, the classical ground for the study 
of the Upper Silurian formation, But T have as yet not been able 
