1879.] 163 (Crosby. 
clay, instead of being a genuine constituent of the shells, is a post 
mortem deposit in their chambers. 
The question, however, as to whether any large proportion of the clay 
actually forms pseudomorphs after the shells of Foraminifera is not 
germane to the present discussion, since it is now generally admitted 
that, no matter how occurring, the red clay has its origin chiefly in a 
source which was at first entirely overlooked; viz., the alteration of 
the pumice and other feldspathic volcanic products, which are uni- 
versally present in this deposit, and even now, in a more or less de- 
composed state, constitute a large part of its bulk. 
Within recent years geologists generally have become familiar 
with the extensive sub-aerial decay of silicate minerals under the 
combined influence of carbonic acid and water; and it is well known 
that the red clay of the soil of unglaciated regions where crystalline 
rocks abound is the chief insoluble product of this rather complex 
reaction. But the fact that substantially similar chemical and phy- 
sical conditions must obtain in the abysses of the ocean is only now 
becoming recognized. The low average temperature of the bottom 
water is an unfavorable circumstance, it is true, but this is offset by 
the finer mechanical division of the material to be acted upon. The 
paucity of life in these abyssal regions is favorable to the peroxidation 
of the iron and manganese, and it is probably to this cause that the 
red color of the deposit is due. Mr. Murray shows that the iron and 
manganese oxides have the same origin as the clay itself, though 
probably coming mainly from the more basic volcanic minerals, as 
olivine and augite, which are never free from these oxides. 
In his address to the Geographical section of the British Associa- 
tion, at its meeting in August last, Prof. Wyville Thomson concludes 
an account of the deep sea clay as follows: — 
_ So far as we can judge, after a most careful comparative exam- 
ination, the deposit which is at present being formed at extreme 
depths in the ocean does not correspond, either in structure or in 
chemical composition, with any known geological formation ; and, 
moreover, we are inclined to believe, from a consideration of their 
structure and of their imbedded organic remains, that none of the 
older formations were laid down at nearly so great depths — that, in 
fact, none of these have anything of an abyssal character. These 
late researches tend to show-that during past geological changes 
abyssal beds have never been exposed, and it seems highly prokable 
that until comparatively recent geological periods such beds have not 
been formed.” 
SF EBS Ee rea sss SSS aS SS ee eee oe 
— 
=r 
a ES es = Se 
