220 
NOTES AND MEMORANDA. 
made by parasitic animals have been mistaken for such. From the 
structure they could not have been related either to Sponges or to 
Hy dr act inice j and still less to Corals; they are truly Foraminiferal, 
and may be regarded as the PalaBozoic representatives of Eozoon. 
Stromatopora occurs infiltrated with calcite or silica, or with its 
structure wholly or in part replaced by crystalline silica or dolomite. 
The author concluded his first section with the characters of the 
genera which have been included in the Stromatoporidge. 
In the second part he noticed a number of facts relating to the 
occurrence of hydrous silicates, of the nature of serpentine and 
loganite, infiltrating Palasozoic fossils and illustrating the mode of 
occurrence and mineralization of Eozoon. Instances of this kind 
were said to be exceedingly common, showing that such silicates, 
whether originating as direct deposits from water, or as products of 
the decomposition of other minerals, are efficient agents in the 
infiltration of the pores and cavities of fossils, and have played this 
part from the earliest geological periods. 
A Microscopical Study of some Huronian Clay-slates . — At the 
meeting of the 19th June, a paper by Dr. Arthur Wichraannwas also 
read, of which the following is an abstract : — Although a considerable 
amount of attention has been devoted during recent years to the 
microscopical study of clay-slates and slate-clays, yet in none of the 
published researches on this subject has any account of the struc- 
ture of the clay-slates of Archaean age been given. The author has 
availed himself of the extensive series of Huronian clay-slates col- 
lected by Major T. V. Brooks in the country' around Lake Superior to 
supply this deficiency. The succession and relation of the rocks 
described have been fully treated of in the work of Hermann Credner 
and the publications of the Geological Survey of Michigan. 
The chief object of the author is to discuss the origin of the crys- 
talline constituents in clay-slates, and at the outset he describes in 
detail the microscopical character of clay-slate, of novaculite or 
whetstone, and of carbonaceous shales and slates respectively, dwelling 
more especially on the crystallized minerals which can be detected in 
each of these rocks, and the nature of the isotropic ground-mass which 
sometimes surrounds them. He then points out that three theories 
have been advanced to account for the presence of these crystalline 
constituents in clay-slates. According to the first of these theories, 
the crystals in question are regarded as the product of chemical action 
in the ocean in which the original material was deposited. The 
second theory attributes the formation of the crystalline minerals 
to processes of metamorphism which have taken place subsequently 
to the solidification of the rocks. The third theory refers them to 
aggregative action going on in the still plastic clay-slate mud prior 
to its solidification. The first of these theories has been maintained 
by G. K. Credner, but against it the author adduces numerous 
arguments, and especially points out the difficulty of supposing an 
ocean capable of depositing from its waters at successive periods 
minerals of such difierent chemical composition as chlorite, actinolite, 
&c. In opposition to the second theory, which has received the 
