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In the long line of cliffs, between fifty and sixty feet in height, called the Barrancas de S. Gregorio, which extend westward of the Rio S. Lucia, the lower half is formed of coarse sand of quartz and feldspar without mica, like that now cast up on the beach near Maldonado; and the upper half of Pampean mud, varying in colour and containing honeycombed veins of soft calcareous matter and small concretions of tosca-rock arranged in lines, and likewise a few pebbles of quartz.
Gypsum in a nearly pure state, about three hundred feet in thickness: this bed, in its concretions of anhydrite and layers of small blackish crystals of carbonate of lime, exactly resembles the great gypseous beds in the Peuquenes range. 22nd. Pale purple and reddish sandstone, as in bed 20: about three hundred feet in thickness. 23rd.
They differ also conspicuously in generally containing several small, or two or three larger, or a single very large concretion of carbonate of lime, as much as 1.5 mm. in diameter. When a gland includes only a few very small concretions, or, as sometimes happens, none at all, it is easily overlooked. The large concretions are round or oval, and exteriorly almost smooth.
Oct. 3. Rise at five o'clock, and start at half-past nine; small plains alternate with a flat forest country, slightly timbered; melon-holes; marly concretions, a stiff clayey soil, beautifully grassed: the prevailing timber trees are Bastard box, the Moreton Bay ash, and the Flooded Gum.
Idealistic systems, still cultivating concretions in discourse, study the first conditions of knowledge and the last interests of life; materialistic systems, still emphasising concretions in existence, describe causal relations, and the habits of nature.
We have, first, To understand what sort of a solution had been employed for the introducing of those various substances; secondly, How those concretions had been formed from such solutions within those bodies of strata; and, lastly, How such concretions could have been formed, without any vestige appearing of the same substance, or of the same operation, in the surrounding part of the stratum.
These naturalists suppose every thing done by means of solution in the mineral kingdom, and yet they are ignorant of those solvents. They conceive or they imagine concretions and crystallizations to be formed of every different substance, and in every place within the solid body of the earth, without considering how far the thing is possible which they suppose.
This volcanic formation resembles one, presently to be described, in Chiloe. This island lies between the Chonos and Chiloe groups: it is about eight hundred feet high, and perhaps has a nucleus of metamorphic rocks. The strata which I examined consisted of fine-grained muddy sandstones, with fragments of lignite and concretions of calcareous sandstone.
Numerous white balls appearing like pisolitic concretions, from the size of a walnut to that of an apple, are embedded in this deposit; they usually have a small pebble in their centres. Mr.
I appeal to the analogy which, in this treatise, he has formed, between the stalactical concretions upon the surface of the earth, and the mineral concretions of siliceous substance. As an example of the great lights, and penetrating genius, of this assiduous studier of nature, I refer to the judicious observations which he has made upon the subject of aluminous earth, in this dissertation.
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