[88]

From conversation & examination of Mr Brown's specimens of agates

Some layers have been soft at [illeg] for lines of crystallization however these & end in point which have reticulated (where crystals minute) the inner layer or indented it, & that layer chiefly is reticulated showing three layers at least at one time soft.

The fine lined or layer structure is not found actually with organic structure; but as in the Cabbage Palm: when shrinking has formed cavities, agate is formed. (bearing relation to forces of cavity on wood) and as cabbage Palm must be of rather short duration so must agate. – we may infer agate is determined by presence of cavity.

[in margin:] then appears struggle when organic – agate structure

When several centres occur in agate, action certainly has been [illeg]; & the layers must be determined by attraction to centre, & not by deposition, for angles [sketch] A depending on centre [sketch] (B)

(B) – not merely a crevice filled up

Hence mass with many centres soft at one time, & layers more concretionary then deposition structure.

Specimens with stalactitic - depending (from gravity because truly parallel) the structure seems certainly modelled from the outside, layers gradually obliterated

[in margin:] If we suppose the points of [illeg] upheaval, some difficulty will be removed: angular or fortification agate

 

[89]

towards the centre. – Hutton states that all mactra come from outside which as far as I could see appears true. – When layer of red oxide of iron is left, may it not be conducted by channel such channels certainly do exist. – yet it is clear there often is centre of apparently indissoluble matter.

May not the two actions go on together wherein cavity good agate structure, or where matter easily removed (as in rather wood which allows of agate structure though sound does not) [insertion:] as was seen in Mr Brown's specimens

Agates probably are determined to center like flints in chalk, but form a small [illeg] of hardening or collapsing mem. Ascension balls of the external parts first. (mem shell of iron balls or concretions) layers are formed when matter is easily removed, [illeg] when organic structure often effected by crystalline action & several centres of attraction going on together.

This kind of attraction may [sketch] go on [illeg] geodes [illeg] form sometimes affected from inside sometimes forms outside cavity [illeg] quarry filled up.

? Particles must very nearly equally disseminated, just sufficient to attract lowered one point. Then when halo was forming particles within might be attracted when

 

[89v]

Agate

Conclusions

Process of silicification moderately rapid several layers soft at one time

Layers chiefly concretionary, determined within by general centres or perhaps by shape of cavity

formed either in cavities or when matter is easily removed

 

Probably Trunks of [illeg] would be [illeg] agate if siliceous particles could move freely. – thus with stone parts may be removed parts left, the concretionary structure determined [illeg] form of outside – slight shrinking towards [illeg] or towards central points.

Henslow strongly insists on aggregation of atoms of iron wh. in some cases certainly seem to have taken place.

{Henslow says there is paper in Annale des Science (1831??) by Brongniart on Agates}

D

+ + Especially in St Albans pebbles, which must be altogether subsequent